I am a GM and I really need advice regarding one of my players.
I play with my group a campaign with FATE rules in a postapocalyptic dark and gritty homebrew world which is mainly horror based, written by a good friend and myself about 2 years ago, which I also GM'd for another group. I give my group a lot of freedom what they do and how they handle the stories since this is my main goal with telling the story of the world itself.
I play with with 5 players (soon 6) while 3 of them take the world and the lore seriously, made beautiful characters and gift me with their own creativity and ideas for the whole plot, I have 2 players that are pretty hard to handle.
One of them is completely passive when he is even present in a session and almost doesn't listen to me as well. He only plays and brings himself into the game when I especially talk to him and ask him what would be the best idea for the sake of the group in the current situation. It is pretty hard to handle this player cause I feel like I talk to a literal brick wall. But he is not the hardest thing.
One of the other players doesn't take anything seriously at all, while I don't mind one joke or too, since I myself make sometimes references as a GM for a little laugh to enlighten in some situations the gritty atmosphere, this player completely overdoes it.
He literally wants to hug and seduce literally everyone, may it be a bandit, or mercenary, mutant or zombie. Or even something bigger I made up. He brings the group into a really dangerous situation and then runs away and lets the other players handle a dangerous situation he himself made up. He asks me the whole time if he can befriend a Mighty Mutant I made up in the lore as one of the dangerous and most hostile thing in the whole lore, and even if a NPC said to him that this is one thing that is impossible, if not completely suicide.
I tried the last session to give him a little kick. I made up a really big threat, a lore heavy mutant that is for the group in their current situation impossible to handle or even to fight. The group saw this mutant and this one player approaches him directly, I gave him several warnings like "a long forgotten instinct that warns him from a mighty predator" and several NPCs that scream to him that he should run since he is literally killing himself.
He wanted to gave the mutant a handshake, and well I have to do what is more lore appropriate, the mutants head split like a lotus blossom and literally bites the player in the whole arm, infecting him with a dangerous Curse.
But well you guess it, even that he didn't take seriously, not even gave any sort of "damn, I messed up" he played it out like it was nothing. And literally shrugs at me when I tried to tell him the danger of his actions.
The group's meeting with this mutant was supposed to be a lore heavy and important for the later story, and it got completely messed up.
I grow tired of this hot mess. Because I made this work and it gone like this, even if I asked multiple times "Can we please concentrate and take it more seriously", this one player completely ignores it and I literally don't know what I can do.
I thought to confront this player outside of session and ask him to take it more seriously 'cause I grow tired, but I also don't want to loose my professionality, if you understand me.
I really thank you for your advice, 'cause I'm desperate and I don't know anymore what to do.
Best Answer
Let This Player Know You Aren't Happy With Their Character's Behavior
There's a second side to this issue - you as the DM need to make it clear to this player that you is not having fun with this. You constructed this whole world for the players and explicitly told them to take it seriously, and you feel like this player is not doing so.
Now, this might just be how their character interacts with the world as a whole, and if that's the case you might need to delve into some player-character separation discussion, but it's also possible they're just not aware that you're looking for the character to take things more seriously.
Adding some real hard consequences to their constant attempts at diplomacy might help - but the reality is, you probably aren't going to be happy with the way they play their character until you tell them that you aren't currently happy with how they're playing their character.
You should also take the advice of other good answers here and make sure he's still interested in playing this game - he might not be, and that would be perfectly fine. Be open to the idea this game might just not be for him. Not every game is meant for every person, and it's perfectly fine to admit that.