In my campaign I have several players that fall really heavily into "My Guy Syndrome" which is to say that they will excuse character actions that sabotage themselves and their teammates because its something that would be in character for their player character.
An example being that my teams sorcerer and rogue would actively be trying to sneak into a building to gather information while my teams monk will run into the building without a care exposing all of them and sabotaging the information gathering because it is in character for them to do that.
While I love having a dedication to your characters persona and I don't want to take that away, (after all that's a core aspect of roleplaying) my players keep on sabotaging each other in this way and no amount of mediation situations or teambuilding exercises that I put them in seem to help. Because their characters now have grudges against one another and its in character for them to snap at one another.
How can I 'delicately' bring this up to my players or how can I as GM create scenarios to help prevent this kind of behavior? How do I tell my problem players that they have 'My Guy Syndrome' and how can I get them to cut it out? The problem is that this behavior is ruining the experience for their teammates by sabotaging team plans and killing team dynamic.
This particular question was prompted by reading the comment thread "what is "My Guy Syndrome" and how do I handle it?" It introduced me to this concept however I noticed most of the solutions were for players and to help players realize when they need to stop this behavior or how to prevent themselves from falling into it. In particular I'm looking to find a solution on how a GM should handle this behavior when they see it happening in their game.
Best Answer
Don't Be Delicate Or Subtle
(But don't go too far and be overly aggressive, or dictatorial, etc.)
It sounds like you've been at this for a year, and from your comment that you and almost all of the other players have little or no experience. This does, I realize, put you in a delicate situation rather than one where you might feel like you can be bold. But the basic fact here is that what you (all of you) have been doing for the last year hasn't been working-- it hasn't been fun for you, for at least one other player and probably hasn't been as much fun as it could be for anyone.
And this is the whole goal of the exercise-- that everyone (including you!) have fun.
So what I would firmly advise is that you gather up your players and dedicate some serious time to a full and free discussion of what's working, what isn't, and how to fix it.
What Is Fun?
But even more than (or at least, before) the subject of "My Guy" syndrome comes up, you might want to start with something really basic: "What is fun?" and "Are we having any of it?" You might be surprised at the answers you get-- meaning that some of the players you're thinking of as problem children might just be having a fine time. RPGs are a big tent-- it's not for me, but there's room in it for slap stick games, comedies of errors, backstabbing, and inter-PC rivalry, grudges and even violence, and so forth. What's a problem is when one or more players want a game that differs significantly from the others.
Or, they may not be content with a game like that, and might be just as unhappy as you. The point is, you won't know until you have this discussion. And that's the context that "Please stop being a My-Guy," fits into: It's not fun.
Make no mistake, by the way: This is a hard discussion to have, because it's really as much about "What is fun?" as it is (now) about "What isn't fun," because you're not having fun. It would be a hard discussion to mediate even if you did not have a personal stake in it, but it's even harder because you do-- your fun is a vital ingredient of the game, too.
It's also hard because this discussion is best had at the beginning of a game, in the planning stages. There are some well-known tools designed for use at the beginning, which you might be able to get some insight from. I do want to warn you that trying to use them straight out of the box, after a year of playing, is probably not going to be helpful, though. These tools are the Session Zero and the Same Page Tool.
As A Side Note...
I'm not 100% sure that what you're describing is My-Guy syndrome. I'm not 100% sure it isn't, either. My experience with My-Guys almost always has an element of anti-social behavior to it-- either anti-social actions in the game world ("Because that's what my guy would do!") or anti-social behavior at the game table ("Because that's what my guy would do!") or, often, both. I'm not getting that vibe, here. But there are edge cases.
You, as the GM, are probably the best judge of whether or not your players are My-Guys. My real point is: Keep your mind as open as possible as you negotiate this process.