Below, I'm going to complain a bit.  If you don't want to read a complaint then go elsewhere.  If you've got something constructive to say that will help me see this in a different light, then please comment.

Our adventuring party recently discovered that one of the PCs had been possessed by an evil spirit for awhile.  While possessed, he did massive amounts of harm to the good guys' interests and, in particular, significant harm to my PC's interest.

I've got no problem with that.  If I had been in that player's shoes, I would have done the same thing to my fellow party members.  Not because either of us are *****, but because we would be roleplaying an evil spirit that is a dick.

As a player, I had figured a long time ago that the possessed PC wasn't right, but I had figured it out based on information that my PC might not know.  So, I told my GM what I had figured out, but I decided not to act on it.  That was out of character information.  So, as far as I was concerned it was out of bounds.  I didn't tell the other players.  But the GM was aware that I was sitting on information that I had chosen not to act on.

However, my character was conceived of, from the time I first made him, as being a superspy like Valerius from Game of Thrones or Nick Fury.  We didn't have SCAG, yet, so the character was a Bard of Lore with the Spy background and a pre-game story which emphasized his concept as a superspy.   He had expertise in Insight and Perception. He had Alertness.  He had expertise in History. His back story had him grow up under a father who was an expert covert operative.  I told the GM on several occasions that my PC was inspired by Nick Fury.

I figure there are things that I, the player, know and things that I, the character knows.  Just as we don't expect Billy Bob the player to be as strong physically as his PC Grim the Slayer, we don't expect Mark, the store clerk, to be as smart as his PC, the Archmage.  We shouldn't expect me to have all the spy skills of my spymaster PC.  I need to do my best to roleplay a spymaster, but, where my own skill set falls short, we've got things like passive Insight checks to cover the gap.  In short, just as I know things that my PC does not, my PC should know things that I, the player, don't.

So, while I'm sitting here deliberately not taking advantage of out of character information, I should have a chance for my PC to discover the same information - especially in light of the fact that he was built and conceived as being an expert in doing exactly that.

The other character was possessed for over a year of in-game time.  Every week, if not every day, my character should have been making passive Insight checks.  

I feel like I got screwed over and railroaded by a GM who was more interested in the story arcs that emerged from everything this evil spirit did, then in letting the heroes of the game have their moment.

My trust at the table has taken a bit hit.  I love my GM, I've known him for twenty years, he's practically a brother to me.  But, I'm still upset and need to vent.

So first and foremost, feelings are valid.  You're entitled to them.  There are also going to be a litany of posts about "You need to talk to your DM", and honestly reading your post, I think you know that.  You describe the dude as a brother to you, and I know you'll have that conversation.

I think for a lot of us older players, we take the concept of metagaming SERIOUSLY, and because of that we don't want to do anything that could be construed as that, but to your point, your character had all the tools needed to be like "Ok, something ain't jiving, what's up?", more so if the player character in question had been adventuring with you prior to possession.  Continuing that tangent, if someone like a Paladin was near you and at ANY point they used Divine Sense near the person in question, that character should have lit up like a beacon.  

You see the light behind what the player did, and I think it is natural to give that benefit of the doubt to the DM.  The flip side to your coin of "You feel like you were railroaded" is "Well, they could have asked to make rolls about it".  It's a vicious cycle.  

That being said, it sounds like you have a lovely table with a bunch of friends who truly enjoy the game, and honestly?  That's a huge thing to celebrate.  Congrats on the long running good times, and I hope they continue.

This is just the cost of railroading. You were forced down a certain path for the sake of the story. There will probably be a lot of comments regarding the problems with PC betrayal and having the PCs work against each other (which I generally don't like but I believe it can be done well under the right circumstances), but it sounds like this would have shaken out the same way if the betrayer was an NPC as well. Sounds like the DM just really wanted his "Agatha All Along" moment.

I'd let him know how you felt. No one is ever finished growing and learning as a DM. Sounds like you have a good relationship so I'm sure he'd appreciate the feedback if nothing else.

Re the insight checks...I don't know. Is it feasible that (assuming you have a party of six including the possessed) he could have fooled the party for more than three days? If you're allowed to roll each day and so is everyone else, the chance of surviving detection is less than 50% after three days (five checks per day with 95% percent chance of failing, which is the highest unless it's actually beyond your ability to succeed and assumes you have to get a nat20). That...sucks. Personally, my ruling would be that you get to roll each time you are given probable cause for suspicion. That might happen twice in one week, or maybe once in a year. Not once a day and not necessarily once a week.

Is it story driven? Probably. The mechanics of 5e aren't well suited to long term deceptions. Hence why you feel that a check each day is appropriate (and I see your logic and think that would be fair) but that makes deceptions that last longer than 3 days unlikely and more than a week pretty much impossible. In an adventure I'm about to do, there'll be a similar substitution, and the players won't be getting daily rolls. They'll get an unexplained check for the week or something. If they notice a problem, they'll get additional checks. I'm a little confused why it ran for a year, but I don't know the adventure etc. I can understand your frustration if you wanted your character to gain the same knowledge as you. If other players twigged as well, I think that the game would have lost the drama and tension. It would have for you, at least, knowing that the "surprise" was coming up but never arriving. Did anyone else figure it out? Or was it just you? If it was just you, then I'd say play along so others can have the fun.

Still, talk to your friend about it and resolve the issue. It's really not worth falling out over a D&D. He probably felt that the story or the table as a whole would benefit from continuing the charade. Let them know that you found it frustrating so they learn not to drag things out when playing with you.

Quote from Spideycloned >>

You see the light behind what the player did, and I think it is natural to give that benefit of the doubt to the DM.  The flip side to your coin of "You feel like you were railroaded" is "Well, they could have asked to make rolls about it".  It's a vicious cycle.

Yeah, this is a big one. I think a lot of us have had times as players where we tried to do something and just flowed with the role play rather than declaring our intentions first, and the DM then went on with the story, and we had to backtrack and say, "Wait, I was trying to Intimidate them there, don't I get a roll?" (or worse, we never get a chance to wind things back.)

Creating a wall between what you know as a player and what you know as a character is admirable, but it doesn't stop you from, as a character, going, "Wait, that seemed weird/didn't add up -- Insight check?" And while I can absolutely sympathize with feeling railroaded by a secret, campaign-altering plotline that lasted that long in game time, I also find it hard to believe that neither DM nor possessed player made zero slipups or dropped zero clues (intentionally or otherwise) that would have given you an opening to ask for that roll without metagaming. Maybe not every day, maybe not on any kind of schedule at all, but at critical or high-stress moments.

The thing about trying to avoid metagaming is that it's really easy to go too far in the other direction, and inadvertently make your character extra dumb or unobservant just to avoid any whiff of impropriety. 

Quote from Wren_the_Munificent >>

  I love my GM, I've known him for twenty years, he's practically a brother to me. 

This bit right here, mate.

Trust your GM. He's given you a long and fascinating story-arc, and set the game characters a cool challenge, and an even cooler nemesis. If you want your characters to progress flawlessly and become world-beating immortals, maybe play Horse Isle or something. For sure, it's probably flawed and railroaded in many ways, and you could rules-lawyer your way into poking more holes in it still, but I quite like it. See where it goes - it'll probably be awesome. :)

It's surprising that no one has found something odd with the possessed character in a year! As the days pass by, i would suspect someone that knows a person well to have a chance to,  even more if that character is an insight expert and spymaster that has an habit of studying people's behavior. I assume your DM has reasons to keep you in the dark. 

If you ever find out why, let us know!

Quote from Wren_the_Munificent >>

The other character was possessed for over a year of in-game time.  Every week, if not every day, my character should have been making passive Insight checks.  

I feel like I got screwed over and railroaded by a GM who was more interested in the story arcs that emerged from everything this evil spirit did, then in letting the heroes of the game have their moment.

My trust at the table has taken a bit hit.  I love my GM, I've known him for twenty years, he's practically a brother to me.  But, I'm still upset and need to vent.

 As a DM I want to start by giving you a good bit of credit. Your ability to keep the meta-game out of the play in the setting is admirable. For that alone I'd be happy to have you at one of my tables (just saying).

The quotes above are where I want to focus. You are feeling crispy about how this played out, and offering solutions that would have avoided it (for you). This is where I'd fault your DM, if I were going to. One of the most important table rules for my games is the agreement that everyone is present to share fun, and that means the fun can't come at the expense of another player's fun. We all have bad days and rough sessions, but a year of in-game play is more than just one crappy night.

While your DM didn't likely intend it, your description sounds like they took away the fun for you by focusing on the fun for this possessed character's player. That's how I'd frame this when you talk to the DM. "I get it, it was fun for <Possessed Player> but it sucked for me as my character is designed to be very perceptive and you never gave me a roll or chance to showcase that. If I'd rolled a 1 I'd be fine because I got a chance. I feel like I didn't get a chance to do attempt something that my character is good at and it kinda sucks the fun out of this plot for me."

From there you can both talk about how to remember that shared fun going forward and move along. He's your brother GM after all. He's going to make mistakes and after you have your say you are going to have the grace to forgive and forget while you all move onward to better adventure.

I'd say a lengthy chat with your buddy is in order, not to be confrontational, or complain or anything, but to better discuss and find out when you could be doing Insight checks, based on what is going on.  You mention that YOU knew what was up and also mention that your character is a great spymaster, while you are not.  That bit kind of points at missed opportunities and someone mentioned that to avoid metagaming, some of us will OVERdumb our characters, beyond what makes sense.  There must have been things the character (possessed one) did or said that tipped you off as a player, so these same things would really ring even clearer to your character, who is more expert in these things.

I'd have a chat with the DM, determine if these events had to occur to allow the campaign to play out, or could you be calling for checks when you feel or think something's fishy.  Playing a spymaster is no doubt tricky, especially if you want to avoid metagaming, but if someone is triggering you, the player, as a little off, a spy would see it like a blazing red light and whistles. You and the DM will end up finding the "line" where you feel something's off and you can allow your character to ask for a roll because he feels something is odd here.

Might do to have a brief OC discussion with everyone if there seems to be any other unrest among the crew.  Make sure everyone's on the same page again, then resume the adventure (and decide if you kill the PC who was possessed out of spite or not lol)

A story involving long term deception by something Evil in a largely good aligned party requires great and highly experienced players, and even when you have them, inter-party conflict is almost never fun.  It leads to squabbling in the real world.  If an Evil spirit possess someone, the signs ought to be evident frequently.  If no clues are given at least once a week, then whomever is playing the possessed character is doing a poor job at best.

Getting rid of an Evil spirit ought to be as simple as dragging the victim to a nearby church and forcing them to drink holy water, or having a Cleric use the Atonement function of the Ceremony spell done as a Ritual.  If that didn't do it, a Remove Curse ought to do the trick.

After a full year of being possessed, the original personality of the character would be gone, or if it existed at all, it would be a case of Multiple Personality Disorder, and who knows how many different people might end up in that character's head.

I knew someone who had MPD.  One of her was a flower, and she communicated only by emotes.  She would indicate that there was a little water spritzer nearby and that she liked to be sprayed with water.  That was the nearest thing you could give her a hug.  Another of her was a young and innocent child, sometimes she was male and tried to be my buddy.  The rest of the time she was a Dominatrix and treated me like I was one as well.  I think about her now and again, and sometimes I cry for her.  Whatever happened to cause that must have been horrible.  There are no Evil Spirits in the real world that I know of, but Evil itself is among us and I see it every time I watch the news.

This isn't railroading as has been suggested. Railroading means that the DM is forcing your actions: it does not mean that your character didn't notice something.

Passive Insight is a terrible mechanic, and shouldn't be used. It removes player agency in actually finding things suspicious. Also, social checks should seldom be used by one PC against another (I'd allow an Insight check on something fun and trivial, like "I don't believe he likes beer!"). If you think your party member is lying based on in-game information, then you can believe they are lying without any need for a check. A game in which Passive Insight exists for a Bard with +13 Insight effectively means that nobody can lie to your party about anything, ever, and get away with it for more than a few sentences at absolute best. It's also impossible for a DM to have to roll Deception checks every time somebody speaks over the course of a year.

The actual issue here is that the DM has put in a story arc that is frustrating for the other players. It's fun being the saboteur: it's really not fun when you have to 'act dumb' and not know what you actually know, especially as it harms the party. I can definitely understand why for a spymaster type character this is doubly frustrating, but ultimately it's more than the DM has initiated PvP in a cooperative game, and then refused to allow you to participate.

Talk to your DM and say "Dude, in D&D the characters are supposed to work together. I get that it's a story arc, but it kind of fundamentally undermines the way the game is played." Since you've known him a long time, he should understand even if his feelings are a bit hurt.

Alternatively, take the role of spymaster and run into it headlong. Have your character become paranoid about the other party members based on the smallest in-game inference that something isn't up with any of them. Then roll a die to see which one he investigates first, so you can't be said to be targeting one specifically. Use your wealth of Bard skills to track their movements, check what they eat, go through their personal possessions when they're out and so on. Compile dossiers on all of that. You could even shift alignment slightly due to your paranoia (which I think all spymasters must have!). Maybe tie it in to seeing some other betrayal in the world ("If it happened to them, it could happen to me..."). If you have access to the Dream spell, start invading their dreams and seeing what they say and do. It could be a good way to further RP as well. But do all this openly: I don't think that tables should have one player knowing anything that the others do not. The game works better when you roleplay not knowing things, rather than simply not knowing them, as you're doing right now (for which I commend you). I find that players enjoy it when they know what's going on behind the scenes, and dislike it when whispering begins. 

As mentioned, this isn't railroading. Appearantly your passive inisght wasn't high enough to catch him. History, perception and alertness isn't really relevant (the observant feat would probably be more useful). 

That said. Just because your character is based on a superspy doesn't mean that your character automatically gets the same powers and abilities. Speaking of, for have many decades had Hydra infilitrated SHIELD right under the nose of Nick Fury? Even a master spymaster can oviously fail. I'm also a bit interested in the fact that appearantly your character has had their interests harmed and decided not to act on that or investigate that for a whole year? Doing that should have given you ample opportuity to figure out, in game, who has behind it without using OOC knowledge.

If the DM used Passive Insight vs Deception, how often did he check? In 365+ days of possession, there could be so many occasions to make a poor Deception and rise suspicions. 

I have a feeling the DM made very few active Charisma (Deception) checks or didn't do any, instead using  a steady DC, a passive Deception or flat out determined the outcome without ever rolling. In any case, the solutions makes no sense to me over such a long one, may be over a short period but not over a year & hundreads of interactions.

To me the DM may have understimated how difficult it is for a stranger to imporsonate someone you don't know without ever rising suspicions even  just once from others very familiar with it. 

Quote from Plaguescarred >>

If the DM used Passive Insight vs Deception, how often did he check? In 365+ days of possession, there could be so many occasions to make a poor Deception and rise suspicions. 

I have a feeling the DM made very few active Charisma (Deception) checks or didn't do any, instead using  a steady DC, a passive Deception or flat out determined the outcome without ever rolling. In any case, the solutions makes no sense to me over such a long one, may be over a short period but not over a year & hundreads of interactions.

To me the DM may have understimated how difficult it is for a stranger to imporsonate someone you don't know without ever rising suspicions even  just once from others very familiar with it. 

 Passive Insight is too implausible to use, and makes all deception impossible. It breaks the world.

Let's say we have two commoners, John and Lucy. They are a married couple. John is cheating on Lucy with Mary while at work. One day they bunk off work together to go kiss in a meadow.

• Lucy asks John: How was work today?
• John says: Oh, you know, the same as always.

The DM rolls Deception for John. Lucy has a passive Insight of 10, and Lucy has +0. Therefore, 50% of the time, John is immediately caught lying and Lucy knows that he didn't have a normal day. So:

• Lucy: I can tell it wasn't. What happened today?
• John: I ate a sandwich and worked on the production line.

50% chance to advance to Lucy now knowing that John didn't have a normal day, and didn't work on the production line. Everything he has said is totally plausible, mundane and reasonable and yet Lucy is on the verge of figuring things out in 2 sentences.

Insight checks should be considered for when something out of the ordinary happens, not as a general lie-detector ongoing all of the time. If Lucy sees John sneaking out at night and asks him where he was going and he says he forgot something at work, roll insight vs. deception or use passive insight (if you must) because Lucy has a reason to believe that John is not telling the truth.

People lie every single day without being caught out about it. How else could an affair happen, or a criminal get away with embezzling? In a world of continuous passive insight, lying is so ineffective that it may as well be impossible. The sheer number of rolls required guarantee failure eventually.

Continuous passive insight is a bit like asking the party to each make a stealth check when they sneak up on something rather than using group stealth. If you ask 5 players to roll a stealth check, it's extremely likely that at least one of them will roll 1-4 and probably fail the check, alerting the enemy. This almost guarantees failing every stealth roll. Dice rolling works well for specific individual actions: it doesn't work well at all for mass actions, or continuous ongoing actions.

One thing to  point out is that the player of the possessed  character has been playing the game since the 1980s.  Almost all of us sitting at the table have.  There is well over 200 years of experience there.  The GM made a secret roll for us only when the player of the possessed character slipped up and did something that would have gotten him caught.  With that much experience under that player's belt, that wasn't often.  The player deserves credit for his roleplay as his goal was to stay undetected all this time.

One thing you could do (run it by the dm) is announce that your super-spy PC is officially suspicious of the possessed character. You can translate your out of game knowledge into an in-character, vague gut feeling. The character doesn't know what is wrong, but feels like something is off about the possessed character, and will begin actively keeping an eye on them and looking for clues as to what is going on with them. 

That way, you can still use your meta knowledge to prompt advancement to the plot while also giving a nod to your character's innate ability to notice things and uncover secrets. 

I'm a little lost as to your point, then?

To my understanding now, the character rarely slipped up and your DM had you and the party do the appropriate checks when they did. Presumably, all the checks failed, so the DM declared that the possession went undetected. This repeated itself for an in-game year (however long that was in real life).

Do you feel that, owing to your background as a spymaster like Nick Fury, the DM should have declared by fiat that you had detected him earlier?

Quote from CharlesThePlant >>

One thing you could do (run it by the dm) is announce that your super-spy PC is officially suspicious of the possessed character. You can translate your out of game knowledge into an in-character, vague gut feeling. The character doesn't know what is wrong, but feels like something is off about the possessed character, and will begin actively keeping an eye on them and looking for clues as to what is going on with them. 

That way, you can still use your meta knowledge to prompt advancement to the plot while also giving a nod to your character's innate ability to notice things and uncover secrets. 

 So, where we are in the game now is that the possessed PC has managed to free himself and report to the party.  So, now, the party knows that the character was possessed.  Over the past year, the evil spirit got away with inflicting massive damage on the good guys' interests and especially my PC's.  Among other things, he wrecked the spy network's (the one I work for) faith in me and released an exceptionally high-level vampire with visions of world conquest loose on my country had another high level vampire's wrath focused on me and replaced at least half of my agents that he knew of with dopplegangers working for him.

As this is not the first time the GM did something like this to me, I did bring it up to him.  We agreed that the kind of character I was running (a superspy) is a bad fit for this campaign.  He will be retiring and be replaced with an Assassin (actually a Shadow Monk/Hexblade).  I do so reluctantly as the other players have stuck with the same PCs from the campaign beginning, have three or more times the wealth my character will start with, and I get a lot of grief from the other players every time I change my character.

Quote from Linklite >>

Do you feel that, owing to your background as a spymaster like Nick Fury, the DM should have declared by fiat that you had detected him earlier?

 Of course not!

But, I do feel that the GM made it a player vs. player contest (instead of a PC vs. NPC contest) even while he knew I was trying to keep player information out of the game.

There are a lot of subtle non-verbal cues, from how the possessed character washed his hands to how he ate his food to how he woke up in the morning, that an expert observer of people should have caught up on.

There should have been a lot more Passive Insight checks.  Instead, there were two or three total.

I understand your sentiment and you, for sure, is entitled to them. BUT, as a DM, I gotta tell you, we might play as "god", but it's tough to think of the implications of every in game mechanic with every single scenario - especially passive ones.

Many answers will fall into the mechanical side and implications of said mechanics, just don't forget that your DM is human too and he probably wasn't even thinking about the implications of your character background on that scenario - especially without external input from you. I think is unlikely that this was an intentional railroading, but more so a failure to perceive your character's innate abilities without player input.

You're friends, the thing already happened, a chat might be good,  but don't let this spoil a good game for you.