My girlfriend Alexis and I saw Vicky Cristina Barcelona this past weekend. Definitely one of Woody Allen's stronger recent films, not quite up there with Match Point, but very good nonetheless. Going into it, I was worried that it would be silly, fluffy, little more than an excuse for the cast and crew to hang out in Barcelona for a few months. The trailers really played up the comedy, and all-out laughs haven't really been Allen's strong suit lately (see Curse of the Jade Scorpion for definitive proof). Thankfully, Vicky is firmly in the genre of dramedy -- it's not afraid to get serious and uncomfortable -- and the laughs are organic to the material. But the reason I'm bringing the movie up is that it turned out to have a lot of useful lessons on writing good characters.
The main one:
Contrasts illustrate character.
Newbie screenwriters, when faced with the task of "establishing" a character early in the script, will often go about the task in the most difficult way possible: by essentially putting the character in a vacuum, presumably so that the reader can observe her directly and (presumably) get to know her quickly. We see Character X looking over her stamp collection, arguing with a barista at Starbucks, applying for a loan, etc. etc. Bad way to do it, if you ask me. It doesn't tell us that much about the character, so it's an inefficient use of screen time. And good screenwriting is largely about efficiency.
So, what's the efficient way to get to know a character? Better yet, what's an efficient way to get to know more than one character at the same time? Easy -- put two (or more) of them in a scene together, create an external event, and show how each of them reacts to it differently. Overly simplistic example: Becky and Cindy are in the kitchen together. A mouse scampers in. Becky shrieks and jumps up on the counter. Cindy bends down and says "Aw, I love mice!" There you go. In two or three lines you've gone a long way towards establishing both of these characters. Like I said, overly simplistic. But Vicky Cristina Barcelona embraces this principle from the very first scenes, and as a result we really get to know both the main characters over the course of the film and, thus, become emotionally invested in the story's outcome. Which is essential to the success of any movie, no matter the genre.
Another good example: The Breakfast Club. Before those kids get put into weekend detention together, they're all living completely separate high school existences in which they'd probably never come in much contact with one another. Does John Hughes show any of that? Nope. He puts them in that library on page three, and starts illustrating the characters for us by having them react to the same situations in their own unique ways. To me, that's much more effective than intercutting. Look how much we learn by observing how each of them treats Anthony Michael Hall, the shy, grade-skipping nerd. Or how each of them reacts when different topics are brought up for discussion: Do they have a lot to say? Do they tell an obvious lie? Do they clam up or change the subject? Every reaction is telling, and the fact that we see so many of them at once is both more efficient and more effective -- because contrasts illustrate character.
And, lest you think this kind of approach only works in talky character-driven films, look how the writers of Speed (Graham Yost, an uncredited Joss Whedon, probably others) develop Keanu Reeves's character in the opening scenes. They need to set him up as a credible action-movie hero, someone we'll believe can pull off all the daredevilry and quick-thinking that will be required of him once the shit hits the fan. So they give him a partner (Jeff Daniels). Both of them are SWAT, so they both have to be pretty tough and clever, but even so, the contrast is obvious -- Keanu is the one hatching all the crazy schemes and undertaking the death-defying feats, while Daniels is clearly a more toe-the-line kind of guy who'd prefer to go home alive at the end of the day. By the end of that opening sequence, then, we know that Keanu is smarter, tougher, more bad-ass than even your average SWAT team member. And thus we can buy him as an action hero. (On the subject of efficiency, it's worth noting that Daniels's character gets used in a variety of other ways over the course of the script. As important as contrasting is, you can't really get away with bringing in a character for that purpose alone.)