There are a lot of mysteries in my campaign and I have been careful about how much plot to release at one time.  One of my PCs is a thief who took his expertise in insight and also has the Observant feat.  His passive perception is something like 22.  Very little gets past him, even passively and he can routinely hit the high 20s or above on his active perception or investigation checks.  They have never failed to spot a trap, a hidden door, etc. and their checks to investigate mysterious locations or crime scenes inevitably results in them finding every single clue.

My problem is with insight.  I have always had an issue with the idea that someone can be a "human lie detector" and thought this kind of thing would have to be more magically based.  My player feels 100% the opposite and that given his high insight it should be virtually impossible for anyone to lie to him.  Due to this dynamic, sessions can bog down with this PC relentlessly interrogating every NPC and asking for numerous insight checks (Yes i know I am not supposed to let the players ask for checks.  My players disagree and, like a bad parent, I still dont know how to stop my players from mostly controlling the game).  And if, later, it is learned that an NPC lied to them, this player is upset and wants to revisit his interrogations of that person so I can show him exactly how this person could have possibly slipped a lie past him.

Current issue a spy who infiltrated their group.  They were uncharacteristically careless in recruiting this person but later the PC doubled back and performed a very thorough interrogation (which we roleplayed through).  However, my reasoning at the time was:  PC is a lvl 8 thief, NPC is a level 15 necromancer cross trained in illusion and divination magic, whose very appearance, voice, etc are all altered by magic items/effects, who has spent the last 30 years infiltrating the courts and castles of the land to spy on nobles and other mages, and was only pulled off of some other high profile work to come spy on the PCs as a favor.  Even with that, I advised the player that some of the spy's reaction to his questions seemed to show some fear and unease, but the topic of the interrogation was dragons (she was a spy for a dragon) so he ultimately assumed it was just fear of evil dragons in general and he allowed her to resume her activities.  She later betrayed them... there was no ultimate downside as they still killed the dragon she was working for and got back all the things she stole from them.  Other than adding some suspense and motivation, this betrayal ultimately cost them nothing.  Yet, in the after math the player continues to remind me that he had tried to interrogate her and that he's disappointed I failed to give him proper credit as the human lie detector he is.

So how powerful do you treat Insight in your games?  With Reliable Talent, I think his minimum Insight check is 21.  Taken literally then nobody short of magic should be able to slip a lie past this person.  And if I stop to roll it to contest with a deception check, the player cannot help but metagame "Aha!  I knew they were trying to lie!"  My handling to date has basically to give him every small lie.  Peasant tries to lie about having seen the werewolf?  Boom.  Merchant tries to lie about how much he paid for an item?  Boom.  Cutpurse tries to lie about needing money to feed his starving children?  Boom.  But on all the big stuff, I only give what I want to give, Insight be damned.  The Player feels that is an abuse of DM power and takes away agency.  He feels he has created sherlock holmes/batman and that if he asks the right questions (and he will ask EVERY question until he finds the right one) then I need to just give him all the plot.  Does he have a point?  Should I treat this as "players will always find ways to disrupt a story" and just give up all the plot?  IN fairness, he did sacrifice other builds that could have made a more optimal character in favor of these traits.  I just think Insight, as he wants to use it, feels too much like a superpower.  A +12 in athletics shouldn't let a PC be able to jump over a 2 story building... I think it is reasonable that all the "normal" skills should have practical limits.

Insight can be a useful tool.  It doesn't give someone the right to take over the table and make everything their way.  The first rule is fun... and at least you aren't having fun with this (admittedly, it's a pain to work around someone building a character like this) even if ALL the other players are... but I would imagine they aren't.  It gets old having one player take over the dialogue for the group for 20-30 minutes at a time and not share the spotlight.  The player may feel like he is a Sherlock Holmes, but he isn't playing with the team.  In fact, you might let him ask a question and then cut over to the rest of the party and see what they are doing.  Make his interrogations take a LONG time while the rest of the group does other things.

Insight doesn't have to rely on Wisdom.  Make some of the checks involving history or religion be based on Intelligence.  If it involves politics or social circles, make it based on Charisma.  

Make some time based puzzles.  Sure, if they can have enough time they can ask all the questions in the world.  In a timed scenario... they can ask a couple of questions but they need to get a move on or bad things are going to happen.

I think that this player sounds a lot like one of mine, with the "I rolled a nat 20 so I can shoot the moon" attitude. I'm still trying to work mine out, but you have a min-maxer on your hands. I would just tell him that he can't detect every lie, even with that high of a passive perception score. Maybe make an NPC he's relentlessly questioning just refuse to talk after a certain amount of time.

NPC is a level 15 necromancer cross trained in illusion and divination magic, whose very appearance, voice, etc are all altered by magic items/effects, who has spent the last 30 years infiltrating the courts and castles of the land to spy on nobles and other mages

Doesn't that mean that the NPC has a killer Deception skill? Why can't you do a skill-vs-skill roll against it? If the thief is +12 insight and the necro is +24 deception, the thief is not going to be likely detecting many of the lies. Also, Insight is not "Detect Lie" or "Detect Magic." If the necro casts a spell that increases the ability to deceive someone, the high insight does not allow to bypass the spell.

Also, don't tell them it is skill-vs-skill. If necessary, use a virtual die roller like an app so the player doesn't hear the dice thunking (if you play in person). Just ask for the insight check and when he gets an 18+12 = 30 and the NPC gets a 10+24 = 34 you say "She seems honest."

Quote from Bartjeebus >>

So how powerful do you treat Insight in your games?  With Reliable Talent, I think his minimum Insight check is 21.  Taken literally then nobody short of magic should be able to slip a lie past this person.  

Excellent. The player has spent a lot of resource to get this ability, so they should be awesome.

However, if the character knows the NPC is lying, what next? Knowing they are lying is only the first part. What are they lying about? Why are they lying? What's their agenda? How can you prove it?

What happens if you confront them? Confronting them without proof ("I know you are lying" is *not* proof) will have implications. You can't just accuse a noble or merchant or ruler of lying and expect to get away with it.

GM: You just accused the leader of the merchant's guild of lying, in front of the entire guild. Since you have no proof, the guild has now blacklisted you. You won't be buying or selling anything with any guild member.

Personal story. One of the more interesting mystery games I've been in was a situation where, due to mind reading, we knew who the guilty party was at the start. Our issue was that mind reading was illegal so we had to find mundane evidence to be able to bring the person to justice. Or manufacture the evidence… which led to an really intersting ingame discussion about the morals of falsifying evidence.

For ideas on this, check out the TV show Lie to Me.  The main character's ability was certainly not a "solve everything" power. Sometimes it was more curse than blessing.

According to the social interaction rules DMG, attempting an insight check requires engaging someone in conversation for an extended time period, at which point you can determine a creature's ideals, bonds, and flaws. Using it to detect lies is generally a passive check, and it's going to be modified if the lie is not all that significant.

Other than being really good at deception, decent magical alternatives include Suggestion and Modify Memory.

I have a home rule when it comes to insight  and perception, I roll for the players and don’t let them see the result and make it clear that if they get a crit fail they will get an incorrect reading for insight. I also only allow one roll and have the highest insight/perception as the modifier with a +1 bonus for all who help with the perception skill

Quote from Grayfax >>

Insight can be a useful tool.  It doesn't give someone the right to take over the table and make everything their way.  The first rule is fun... and at least you aren't having fun with this (admittedly, it's a pain to work around someone building a character like this) even if ALL the other players are... but I would imagine they aren't.  It gets old having one player take over the dialogue for the group for 20-30 minutes at a time and not share the spotlight.  The player may feel like he is a Sherlock Holmes, but he isn't playing with the team.  In fact, you might let him ask a question and then cut over to the rest of the party and see what they are doing.  Make his interrogations take a LONG time while the rest of the group does other things.

 

 This is a really good point. As the DM, you do have final say on this kind of thing, and you definitely have the right to pan away from him for a while. If his questions are rationed frequently enough, he may start being more circumspect with his checks. 

 If it keeps up, you may just want to have a talk with him. Set out exactly how you feel Insight should be used. Reassure him that the skill won't be useless, but also that it doesn't mean he gets to know everything about everyone.

I would be using a passive insight  (10+his modifiers) and then roll deception for the NPC for how well they pull it off. Then use this to tell the player how the NPC comes across to them. This removes their "knowledge" of how well they did ("I rolled 19, +12, so that's 31, so whatever the DM says must be true!") - you roll a dice behind the screen, then tell them the NPC seems truthful. what did you roll - low means it is true, high means it might not be! If the player is interrogating, roll with disadvantage for the NPC because they are actively looking for lies.

For traps, it's all well and good seeing the trap - it's another thing to know how to get around it. I also tend to avoid "see it or get hurt" traps - I don't feel they add anything to the experience, and only make people slow down to look for traps in every room. I prefer to have the traps as encounters, where the trap goes off and now they have to do something in a set number of turns or something worse happens - like getting flushed out of the temple, or dumped into a cage, or something like that. Traps can be perfectly concealed - they can also be magically triggered. High perception only lets you perceive what's there - they might see scorch marks on the ceiling, or that the walls of the room are damp and mossy. they won't see a magically triggered trap which starts when the door closes.

Look up physical indicators for someone that is lying. Use one or multiple of those when describing the results, vs just outright saying whether an NPC is lying. (Which you already seem to be doing based on the necromancer interaction.) Nothing in the insight skill description says they are sure of a lie, just that they can pick out mannerisms and body language. Regardless of how high the skill is, it's not the Zone of Truth spell.

To back up and expand on what a lot of people on this thread are saying, I use a lot of physical signs of lying- but because people are widely varied, these signs of guilt can also just be signs of nervousness or a habit of the person. For example, refusal to make eye contact can often be seen as an indication of guilt- but it can also mean that the person is shy, or they're scared of the adventurer, or they're hiding an entirely unrelated thing, such as someone accused of murder trying to hide that he was sleeping with someone who wasn't his spouse. I like what you did with the spy- you gave the signs but the player interpreted them wrongly, which happens even to skilled interrogators sometimes. I also occasionally throw in context that the characters would know, based on either their backgrounds or their past actions in the game that they might not remember off the top of their heads if they roll well or just barely better than the opponent. "You don't see any obvious signs of deception, but you know that green dragons are master manipulators."

Also, keep in mind that you don't have to let this player do all of these long interrogations- an NPC might well get fed up with all the questions, even (or especially) innocent ones! I just ran a mystery side-plot in my game and the NPCs had certain lines they would not cross. The mayor hired the PCs to figure out what had destroyed the crops in the town, but when they were interested in interrogating his son (and other students), he shut down and directed them elsewhere. The guilty party (a person who became a warlock to get revenge for how the town drove out his child) was clearly grief-stricken and he became easily overwhelmed when they asked about his child, to the point the PCs didn't get much out of him but still thought he seemed sympathetic. One of the people who provided knowledge on contract with demons refused to talk much more to the PCs for fear of becoming infected when she realized the whole town was being corrupted. The teacher for the small town refused to answer any specifics because he knew the town was likely to blame the newcomer. Overall, the pieces they gained from each person allowed them to discover the guilty party and devise a solution. No NPC has to just stand there and take the PC's questions, especially if the PC comes in heavy-handed. Then, once the PC maxes out the NPC's willingness to cooperate, you can make that clear and move the plot along.

You can also pull in the other players during an interrogation if the whole group is interested in talking to a particular NPC. The group in which I play switches off on who takes what position based on what we know about the NPC we're questioning and our characters, but generally somebody takes a softer approach, another person provides a hint of intimidation, a third provides magic that enhances interrogation, and the fourth snarks/makes strange comments to keep the NPC off-guard. Then based on which combinations are working best, the people in those roles take the lead unless the NPC is especially relevant to one character's backstory and plot, in which case that character pretty much always takes the lead. The group should rarely just be standing there, although that doesn't mean everybody has to do a lot on the same interrogation as long as people switch it up over time.

So I think it's fair to point out that if a character can have that high of an insight, it's perfectly fine for another character to have that high of a deception, the spy NPC you mentioned is a perfect example.

Of course when it come to more ordinary people I have a simpler solution. Make sure your NPCs give lots of information and mix truth with lies, or have multiple lies in same sentance.. You mentioned a peasant lying about seeing a werewolf. What if instead of the peasant saying "I didn't see the werewolf," they said "I was walking from the fields to go home, and I didn't see the werewolf." Sure the peasant is lying about the werewolf, but the big lie is they weren't leaving the field, they are actually cheating on their spouse and were leaving their mistresses house. That's the lie they are concerned with and the one that 'pings' the insight. So tell the player the peasant is lying about their walk home. Let them chase information that isn't relevant on occasion. This may help them realize that insight check aren't the be all and end all.

Similarly with the merchant, why are they lying about how much they paid for an item? If your character wants to be Sherlock homes then maybe they pick up on the fact that the merchant is about to lose his business and that's why he's nervous, and maybe yes, they still know about the lie relating to the item price, but maybe this changes their focus. This does mean you have to spend more time making backstory for your NPCs, because it might be hard to come up with this on the fly, but it could make your game more interesting overall.

Finally you mentioned

The Player feels that is an abuse of DM power and takes away agency.  He feels he has created sherlock holmes/batman and that if he asks the right questions (and he will ask EVERY question until he finds the right one) then I need to just give him all the plot. 

So don't give him time to keep asking all the questions. Find ways to interrupt interrogations. Many NPCs work for other NPCs. Have the boss come over and force the first NPC to get back to work. Are you interrogating a guard? Have the other guards show up and attack. There's all kinds of other NPC and situations that can interrupt before they can ask all the questions. That can also help your world seem more like a living world. Not everything is waiting for the players to make up their minds. Somethings just happen whether the players act or not.

GGG's comments about multiple lies reminds me of an old Hercule Poirot rule of thumb from Agatha Christie's mystery novels. Poirot, her "Sherlock Holmes" sleuth, once said to his assistant Hastings, that "everyone lies" when you question people about murder. Surely not everyone, Hastings objected. "Everyone," Poirot responded. The question is, which lies are material to the case, and which lies are about something else.

GGG's example is exactly what he used... a person lies about his whereabouts on the night of the murder, not because he was the murderer, but because he was cheating on his wife, and doesn't want her to find out. His lie is not material to the case, in fact it has zero to do with the case, but the lie will trip up the investigator because it is a lie, and it won't bear up under scrutiny, and then you end up going down blind alleys trying to find out why Col. Mustard is lying, because you think it means he is the killer, when in fact he is just having an affair with Miss Scarlet, who is also lying, and neither of them had anything to do with the murder.

In the context of the case when Poirot made that statement, they had a group of suspects around a dining room table, and all of them had told a lie. Poirot knew it, just because he knew people lied, and he said to Hastings that their job was figuring out which lie actually mattered, in that it implicated the murderer, and which lies were, well... just lies.

I can't remember which TV show it was, possibly Frasier... it was a long time ago, but Frasier was a psychiatrist so it sounds right... in which the main character said, "People lie all the time, every day, about many little things." They lie to avoid hurting someone's feelings ("You did a great job up there on stage tonight"), or to avoid an argument ("No honey, that dress doesn't make you look fat"), or to get what they want ("I'll give you this if you do this for me first."). But most of all, he said, people lie to themselves ("This year I really will do my New Year's resolution.").

So GGG is right -- it's one thing to tell a player, your character thinks based on body language that this person is hiding something. But what's the NPC hiding? Something about the murder you're asking about? Or something about the affair with the farm girl in the field next door that he doesn't want his wife to find out about, and about which you couldn't give the slightest rip even if you knew it?

Quote from Sul_Khatesh >>

I have a home rule when it comes to insight  and perception, I roll for the players and don’t let them see the result and make it clear that if they get a crit fail they will get an incorrect reading for insight. I also only allow one roll and have the highest insight/perception as the modifier with a +1 bonus for all who help with the perception skill

 
I think that failing an Insight check gives incorrect information. Not just rolling a natural 1.