This is some follow-on from my musing about too many protagonists.
When we sit down to play a role-playing game, there are three to seven of us (more or less) who want to make a narrative about a bunch of characters. Great. Depending on the game, one of us might play a more general role, wrangling all the walk-ons and antagonists and such, freeing everyone else to really get into the protagonists’ roles.
This doesn’t actually say anything about the nature of the protagonists’ relationships with each other.
A lot of games do, however, come with expectations about the protagonists’ relationships with each other; very often, that expectation is that they will form a group of some sort, working towards common goals, maybe not always on the best of terms, but pulling together when the going gets tough. This is an assumption that I think is inherited from D&D. It’s not a bad one, either, as long as games are clear about it.
You’ve got to manage expectations here, just as in any other part of a game. In the absence of instructions, people will often act in whatever way they’re most used to. For gamers, that’s often towards the D&D model: a motley bunch of misfits who all want something in common, and work together. And that’s not right for all games. Consider, for example, Smallville: in that game, you really need a group who are thematically aligned (not so much a motley group of misfits, unless that is the theme), but whose interests are all at cross-purposes. And once you know that, the game clicks. ((For the best example I’ve ever seen of helping everyone at the table align their expectations, allow me to recommend Primetime Adventures.))
Now, as a final thought, you don’t have to make that management of expectations explicit, necessarily. Certain things in a game will encourage certain behaviors. For example, a two-way divide between the participants, with one “GM” and many players, will align the players like a magnet aligns iron filings. It will encourage the players to act as a team, whereas the absence of this divide (as, say, in Fiasco) helps players all see themselves as peers who can have goals at cross-purposes. ((But, you can counter that, as Apocalypse World does, with a clearly defined role for the odd one out that encourages them to sow chaos among the others.))