Jane Espenson has some great advice for screen writers up on her blog:

I've talked a lot about making sure that, as often as possible, your jokes are used to express character. It occurs to me now that there's no reason to limit this idea to jokes. You may find yourself writing the dourest darkest drama unleavened by any humor (although I hope not -- everything's better with some funny in it), and you should still make an effort to make sure that the lines your characters speak are constructed to illuminate who they are.

In order to accomplish this, it helps if all of your characters have attitudes, actual opinions about what's going on in the story. Got a character who isn't invested in the story? How about making him SO uninvested that he's advocating not going on the adventure at all?...

[I]f a character doesn't have a logical reason to take a certain action or advocate for a certain position, then give them an emotional reason. Like, for example, antipathy toward the hero or a desire to sleep with the villain or an unconscious identification with the victim... find a way to keep them invested.

Characters who care deeply, even if it's for bad reasons, are going to make every scene about themselves. Which you know, if you've ever been in the middle of a group of people who think every scene is about themselves, is very, very good for drama.