I was watching the actor commentary on The Social Network this past weekend, and one of the many interesting tidbits that comes up is the fact that Andrew Garfield and Jesse Eisenberg each read for the other's roles at one point. In fact Jesse mentions that it was quite interesting to watch Garfield's performance, as he realized that Garfield was making choices that never would have occurred to him.
This reminded me of something that a lot of writers either forget or never learn - the script is just one aspect of a very collaborative process. An actor will put the screenplay through their own "process" as they attempt to build their performance. Sometimes they draw conclusions about their character that line up with your intentions, sometimes they won't. They'll stick to your script (mostly), but there's no guarantee that their view of the character will be precisely the same as yours.
But the thing that allows two actors to make different choices that are both "right" for the film is that as written, the characters have depth and complexity. Andrew Garfield's Eduardo might be totally different from Jesse Eisenberg's Eduardo, but they could both be valid within the script. Good writing makes this sort of complexity possible, as it provides a wealth of opportunity for an actor to draw upon.
Bad writing reduces characters to little more than chess pieces manipulated though the game board that is the plot. Those sorts of characters are rarely interesting for actors to play, and I guarantee they're not much fun for people like me to read. Now, this is NOT an invitation to write a long opening sequence that tries to tell your character's whole life story via flashbacks and therapy sessions. Instead, it's a reminder to approach every scene while keeping in mind each character's POV, motivations and history that they bring to that moment. Find little things you can do that resonate on a character level. (If you're really good, you can find little moments that do this, and go in line with the themes of the story.)
I'm sure there are people who could write entire books about this. I think an easy thing to boil this down to is: "Write characters who actors would want to play." Writers sometimes get so caught up in their cool story or the awesome twists that they're sure no one will see coming that they totally forget to give the actors anything beyond executing those twists.
Characters need to be more than a means to plot's end. Good characters can get you good actors, and good actors can often get you a successful movie.