Character important details? I don't necessarily mean about backstory, but for example, somehting that would effect their character in a surprising/unknown way? 

I've had several instances of ideas of just such things, but i always struggle with the whole "I want to ask them if they're ok with this since it effects their character" vs "But if I tell them it somewhat spoils the story for them, or may even tip them off to something unintentionally"

Ironically enough i can't think of any of the current situations as i type this, but i'm tired from work, so i'll use a stand in.

There are a pair of 'demon-like' characters in one of the forgotten realms stories... i forget which, but they're either brothers or friends. One of them has this condition/curse/whatever where every so many days, he has to put his lips to some other human(oid)s and transfer some demonic larvae into another innocent soul, or die horribly himself by becoming some bloated monstrosity.

It starts with him coughing as the 'warnign sign' that he has to do it soon. And he ultimately kills multiple people in this way by transferring the larva from his own body into the body of another, thus preventing his own death... temporarily, and those poor people end up becoming horrific abberations that go around killing once their transformation completes.

Something like that. Part of me feels the 'always ask' route is the right way, but at the same time....

My worry about talking about it is always that it will make the event or scenario less impactful to the player, since they will come to know things about it. Among other things.

I'm not clear on what you mean. Perhaps if you could say the actual situation, it would be easier. Are you saying the PC is the carrier? Or that you want the carrier to kiss the PC and infect them?

Generally, I'm uncomfortable with making permanent changes to a character without their permission. Obviously, they can die in combat, but I'd be reluctant to force them to become a wererat or something. If they were attacked by one and I decided it might be interesting to pursue that avenue, I'd ask them between sessions and if they don't want to do it, I'll just have the bite not be effective or something.

You could also just ask generally. Are you happy with your characters undergoing transformations, being slowly killed by diseases, having agency removed, etc? Ideally, that'd be part of session 0. If they agree in general terms, you can continue.

The major problem is that people can get attached to their characters. I'd get permission to do anything that would permanently alter them (death in combat is always a given permission by the nature of the game), even if just in general terms.

Your example isnt far from the mark, and is an example of another. To be clear, this isnt a situation that is happening (therefore i cannot day the actual situation :P). I just wanted some general feedback, and the example above is fro ma forgotten realms novel or series i read since it was all i could think of at the time. But aye, more or less as you described. 

Using your wererat example, if this was something i thought of AHEAD of a current situation. As in  - I don't have them fightign wererats, and then go "Oh maybe I should infect one!" but rather as i'm brainstorming for the future whether the game has already started or i'm just thinking of scenarios for later in the game. Thats what I mean by worrying that by saying X might happen that it technically tips the player off to potentially more than intended.

This is a fact specific question that is going to depend both on how you implement your idea and your individual players.  There is not really a right answer - you are just going to have to play it by ear.  However, while I do not think anyone can tell you the answer, there are a few things you should consider:

1. Are you being heavy-handed in the character change or is it coming organically out of the story?  Being heavy handed makes players feel bad - it feels like a cop out on the part of the DM to solve a problem they do not know how to solve.  By way of example, I was playing an Oathbreaker Paladin once, who broke his oath to unsuccessfully save his wife.  In the 5th session, the DM had a god physically manifest, provide a letter from the wife absolving my character of guilt, and say “and now you are mine—you are no longer an oathbreaker!”  That is an extreme example of deus ex machina heavy handed DMing (he was a really bad DM), but it still goes to show the kinds of things you should avoid.

A more organic example would be giving the players clues to “solve” one of their personal objectives over the course of several sessions, and then having some element of that objective be “solved” without their involvement because they chose to ignore prolonged clues.  That is still you changing part of their character’s story, but done so in a way that grew out of the player’s choices, not the DM forcing them on the player. 

2. Think about your motives.  Are you doing this because you do not like a part of the character and want it to change (not a good motive), or are you doing it for the sake of the story, the player, or the character’s development (fine motives)?

3. Is your individual player the kind of person who wants to control every aspect of their character or do they prefer to let their character react to the story?  Are they flexible and able to quickly adapt to potentially life-altering events, or are they going to need to be prepared ahead of time or risk struggling with the roleplaying?  Think about what would work for that individual character—and what might work for one player might not work for another player within the same party, so you need not treat everyone exactly the same.

Quote from Raiketsu >>

Your example isnt far from the mark, and is an example of another. To be clear, this isnt a situation that is happening (therefore i cannot day the actual situation :P). I just wanted some general feedback, and the example above is fro ma forgotten realms novel or series i read since it was all i could think of at the time. But aye, more or less as you described. 

Using your wererat example, if this was something i thought of AHEAD of a current situation. As in  - I don't have them fightign wererats, and then go "Oh maybe I should infect one!" but rather as i'm brainstorming for the future whether the game has already started or i'm just thinking of scenarios for later in the game. Thats what I mean by worrying that by saying X might happen that it technically tips the player off to potentially more than intended.

 I understood that it was a hypothetical, I was just unclear as to what kinds of changes to the character you were referring to. Personally, I don't plan changes to the characters, I plan changes to the world, and let them react to that. So, I won't plan "in session 43, I'll have the Wererats bite Boris the Bewildered and turn him into a Wererat. Instead, I'll have a rough outline of how I expect the session to go, and if it organically comes up. I should have, through session 0 and knowing the players, figured out who will be on board for being turned into one (not because I've asked if they are willing to become a Wererat, but because in session 0 I've asked things like "Are you happy to have your characters have changes inflicted on them? What if it's permanent?"), and then I'll threaten it in some way to get a reaction out of the players. Between those, hopefully I'll have some insight as to who is happy for that to happen to them and who would resent it. I can then shape events to ensure it doesn't happen to someone that would resent it. If they're all captured, it's easy - I can pick who goes first. If it's just one and they don't want to be turned, I can alter the story. their party members find a secret passage that brings them to the chamber in time to rescue them, or an NPC intervenes and brings the party (or helps the PC escape) or something.

So that's how I'd do it. In session 0, just ask who is happy to let whatever happen to their character as the plot entails (doesn't need to be specific, just enough that you can get a feel for what is acceptable and what isn't). Then just a quick recheck when it comes up (if feasible, it might not be, but that's why you asked in session 0 so you have the permission so this isn't a must, and you can inform them of their status when it becomes relevant). Also, be prepared for a way out - perhaps a cure or something, in case they've changed their minds. 

Always ask. Tbh I wouldn’t want that happening to my character. Maybe in like a Gothic horror game where I was expecting something like that.

If a player doesn't want anything happening, his character should just stay home. To me, part of the thrill and awe of adventuring is to discover what happen to your character, good or bad. It's risks coming with the job basically.

Poison, Trap, disease, insanity, curse, possession, polymorph, death or averse effects are all part of the game and is something that can possibly happen, and i expect players should know and accept that. If something detrimental happen, characters must look for ways to resolve it in game.

PS I use D&D Safety Tools in my campaign to discuss themes and limits.

Quote from WoodsGirl0402 >>

Always ask. Tbh I wouldn’t want that happening to my character. Maybe in like a Gothic horror game where I was expecting something like that.

Yeah, unless you had a more general discussion back in session 0 like, "The nature of this campaign is such that curses, transformations etc. are on the table for every character if the situation arises. Are you OK with that, and are there are lines you don't want crossed?", then you definitely want to have it before proceeding down that path mid-campaign

Bottom line is, it's not your character, and you shouldn't be making fundamental changes to it without the player's input 

Quote from Linklite >>

You could also just ask generally. Are you happy with your characters undergoing transformations, being slowly killed by diseases, having agency removed, etc? Ideally, that'd be part of session 0. If they agree in general terms, you can continue.

 
This👆I would want it discussed in Session 0 so I could consent in advance and then enjoy the surprise. But that’s me. I also don’t get nearly so attached to my characters because any one of them could die any session, and I started playing in 2e. Back then life was cheap. We played them 3 at a time and had a stack ready to replace losses, so I’ve developed a different type of relationship with my characters.

The other thing I want to point out is that, as long as there was a potential way to undo the changes to my character I really wouldn’t mind at all. If it were possible to go on a quest and be cured of lycanthropy then no harm, no foul. Make sense?

I hope that helps.

I would say in things that will fundamentally change that players character, their are three scenarios:

1.  You have been playing with the player for years.  You have built up significant trust and they have indicated that you as the DM are free to do as you wish, because of the trust you have earned.  In which case, it's probably ok not to talk to the player, however a simple "I have a cool idea of something to do with your character.  Are you ok with unexpected changes?"  If they say yes, then go for it.  But always give the opportunity to change back.  If it is like the Wererat situation, let there be the possibility of a finding a cure.  

2.  You've discussed these things in your Session 0.  You've said "There could be things in the game that can fundamentally change your character.  Are you ok with this?"  If they said yes, then you probably don't need to talk to the player.  Just again make sure that the changes can be reversed if they so choose.  That doesn't have to be easy, but at least possible.

3.  You've neither built up significant trust with the player (are mostly new to playing with them) and you didn't do a Session 0.  In this case, simply ask in vague terms.  Say "Hey, I have an interesting idea for something to do with your character, but it might lead to significant changes to them.  Are you ok with that, or no?" If no, then don't do it.  If yes, go for it, but again leave the opportunity to "fix" whatever the change is if they so choose.

 If you are going to change characters and give them demonic curses as "surprise," then they need to know that that is the type of game they are in for. You don't need to tell them the details, but if you plan to run this type of game, then you certainly need to give your players some type of warning in advance that some of their player agency might be taken away. This may be a hypothetical situation, but were you to actually put it in a game, you would need your players to be comfortable with the odd way it is spread, and that their character might be turned into a horrible monster.

In short, if you want to mess with your player's characters, you need to communicate clearly that you are going to be doing that, and the extent/scope you are going to be doing it to. Honestly, if you were to use the above hypothetical example in a campaign with someone like me without a warning,, then I would not want to be sitting at your table again. To be clear, there is nothing wrong with that style of play with demonic curses and stuff. However, there are many people who do not want to play in a game like that. so I think it is incredibly important to warn your players before starting the campaign that you might be messing with their characters and agency. And if they don't want to play in a group like that, either adjust that part of your play or just don't play with them.