There are lots of ways for a GM's campaign to go down in flames. One of those is when the player characters start conflicts between themselves.
Sometimes this can be a good thing, a running joke, some casual sniping or practical jokes between the PCs. At other times, though, PC conflict can completely derail the campaign as the focus is on the conflict between the PCs, not the GM's campaign.
One of the classic examples of this in a typical fantasy campaign is mixing Good and Evil characters in the same group. That can go wrong in many ways, and right in only a few. But PC conflict can come out of in-game actions, too, creating a rift between formerly friendly characters.
Here are some tips on handling player character conflict.
Screen Party Composition
The Good and Evil conflict is one that can be pre-screened by the GM, along with other conflicts more specific to the campaign world (racial or social conflicts, for example).
A GM unwilling to take the chance on this sort of conflict can simply tell players the acceptable combinations for the party. All Good characters, for example. In many cases that only makes sense...a campaign where the party must defeat a evil wizard is more likely to go the way the GM wants with Good characters. A GM who allowed an Evil party into a campaign like that would need to be prepared to be flexible, as the PCs tried to take over the evil wizard's operation rather than defeating him.
Some GMs are not comfortable limiting their players' options, but it's just good sense to make sure everyone understands the sort of game they'll be playing and the sort of characters that fit. That helps establish the social contract of the game.
Provide Reasons To Cooperate
If PC conflict does happen, for whatever reason, a GM can provide those characters with a reason to cooperate.
The classic example of this is the two deadly enemies who are handcuffed to each other and need to work together to survive. While that specific example won't work in most campaigns, the concept is the same.
In a recent campaign I was playing a Good paladin, and another character was playing an Evil necromancer. The GM was concerned about the potential for conflict, so he set up the plot such that the paladin became in charge of the redemption of the necromancer, who in turn needed to cooperate with the paladin to avoid prison.
Provide A Common Enemy
PCs who are in conflict can quickly ignore that conflict in the face of a common enemy. That enemy might be the evil wizard they're supposed to defeat, or someone else, but the common enemy gives the PCs a reason to cooperate.
A GM using this technique should be aware that the PCs will likely cooperate only as long as that common enemy is a problem. Either pick the main objective of the campaign as the common enemy, or use the Onion method. In the Onion method, defeating the common enemy reveals someone else behind her, who then becomes a new (and more powerful) common enemy.
Also be warned that this technique will not end the PC conflict. It'll simply put it into perspective, so that it doesn't derail the campaign. But expect the PCs to continue to work against each other in small ways that don't harm their chances of overcoming their common enemy.
Remind Players Of Their Reason For Playing
Part of the responsibility for upholding the social contract rests with the players themselves. They've agreed to play a particular game; while part of that game is playing their characters, another part is helping everyone to have fun.
In the paladin/necromancer example, the players themselves could have worked out ways for the characters to cooperate and coexist. A paladin's charge is to uphold the Good, not to slaughter every Evil character she encounters. A necromancer animates dead bodies, but is that bad? That depends on how theology works in the campaign world. I left my paladin deliberately ignorant of that, so that he wouldn't be certain enough to start a conflict with a necromancer just because of the sort of magic the necromancer uses.
By the same token, the necromancer's player would have to see that Evil doesn't mean laughing maniacally and slaughtering innocents at every turn.
Players, by rejecting stereotypes and fleshing out their characters more, can avoid many sorts of PC conflict themselves. The GM techniques we've looked at can help rationalize this in-game.
Let It Happen
A GM comfortable with playing things by ear can allow the PC conflict to become a major part of the campaign. Perhaps the party splits into opposing sides, each working against the other. Each works to gain power in the campaign world so that they can overcome their rivals.
This is a difficult thing for the GM, but can be very rewarding for the players. It would be better, though, if the GM knew up front that this was the sort of campaign she was going to run, so that the social contract everyone agreed to had that expectation in it.
Remove The Cause
Finally, if you've tried everything to end the PC conflict, and it's seriously interfering with the campaign you want to run, feel free to either kill off the most offending PC, or switch her to NPC status and bring her back as a recurring villain. The player might not be happy, but the entire group probably hasn't been happy with the ongoing PC conflict.
Getting things back on track might just be worth it.