If you listened to the David Mamet broadcast from the last post, you noticed that he talks about the one central question in every movie watcher's head: what's gonna happen now? It's not about what's happening right now. It's not what happened in the backstory. It's a question that pulls the viewer forward into the next scene, and a good screenwriter knows how to work that question to his advantage.
Say your script is about a man who cheats on his wife but still loves her. Many screenwriters will go about this by having a love scene between the man and his mistress, with gasps of pleasure punctuating lines of dialogue like, "But, Bob, you do love me, don't you?" *gasp* "Of course, Martha. I love you with a passion I can't even understand!" *moan* "But your wife!" *frenzied panting* "I love her too, I can't deny it!"
Okay. We got it. Yeah. What's funny about that scene is that it actually works better without the dialogue, and not just as a sex scene. Bob and Martha have at it, and, when they aren't commentating, we, the audience can simply examine what really matters between them. Suddenly the scene becomes a lot more flexible. In the hands of capable actors, it can mean a lot more. And if the scene is successful, we'll be asking one question: what's gonna happen now? Will Bob leave his wife? Will Martha cause problems? These issues frame the material so much better than dialogue that tells us what we're supposed to be thinking about.
The screenwriter molds these questions by carefully choosing scenes, actions, and visuals. What if we cut to Bob at his child's baptism next? What does that say? Or Bob's wife alone in her hospital bed? Whatever she's doing will take on greater meaning when contrasted to the scene before it.
I've mentioned before that the screenwriter has a very important role at the beginning of a long, collaborative process. We don't tell actors how to act. We don't tell cinematographers how to shoot. But we create the potential for them to excel and tell the story themselves. We open up an empty space. For me, this is the big difference between screenwriting and novel writing. We don't get to describe the tiny nuanced shifts of a character's psychology. But we do get to build a series of scenes that bring it all home for an audience.