Many times, I find the situations that bring PCs together to be artificial. In my experience, you often find groups of player characters who, personality, alignment, or goals-wise, are only sticking together because their players are seated together physically at a table playing the same game. I recall a horrible campaign a friend and I joined several years ago where one of the players immediately told us he would kill our characters if we made a paladin, because he refused to adventure with paladins. My buddy made a paladin immediately. Luckily, the guy was as much of a sniveling coward in game as he seemed to be in real life, and his wrath was limited to passive aggressive behavior towards my buddy's player.
Even among friends, I find that characters are often played with such opposing goals and personalities that the only reason they stay together is because their players are seated physically at the same table playing the same game. This is especially true in D&D type games when someone wants to play a cleric that is actually religious amongst the typical drinking, looting, whoring player character party. Crack open any White Wolf book from the Old World of Darkness and you're going to find that antagonism between character types is built into the setting, with most of the Clans/Tribes/Traditions disliking 90% of the other Clans/Tribes/Traditions in the same game.
Usually, when these sorts of conflicts come up, everyone is put in a slightly awkward situation. Usually one or both parties have to "look the other way," or the DM has to gloss over the conflict. After all, if it continues to be role-played out, one character is going to leave or even end up dead. The cleric will not abide the thief murdering townspeople. The witch hunter is going to have a hard time adventuring with the hedge mage. Nobody wants to back down, and the DM doesn't want to pick one character over the other, especially if both characters, individually, are suited to the campaign.
The new RIFTS game I'm in has some of this conflict already, and although I am having fun with the campaign, it bears examination.
Our party: Wired Gunslinger, Wild Psi-Stalker, Wild Dog Boy/Wilderness Scout, Body Fixer (me), Ley Line Walker, Elemental Fusionist (Fire/Water)
-The Psi-Stalker should constantly be salivating over the two spellcasters in the party, especially when he hasn't had a fix. This has been glossed over so far.
-The Psi-Stalker attacked one of the Body Fixer's patients while he was trying to perform surgery on said patient. This lead to PvP combat, with the Body Fixer tranquilizing the Psi-Stalker with drugs. Meanwhile, the Dog Boy got into a brawl with said patient's companion in the lobby of the Body Fixer's clinic. Logically, the Fixer would never work with these two again, but that would mean someone would have to make a new character, so the Stalker and Dog Boy are merely banned from the Fixer's clinic.
As I said earlier, I am having fun (I play the Body Fixer), but the incident has got my mind on just how artificial some of the relationships between player characters can be. Everyone wants to play their character, but there is a bit of a fourth wall holding the player characters together, no matter how at odds with one another they might get.