Discussing the very talky opening scene of The Social Network, Aaron Sorkin makes a key point about how writers dole out information:

We started at 100 miles an hour in the middle of a conversation, and that makes the audience have to run to catch up. […]The worst crime you can commit with an audience is telling them something they already know. We were always running ahead.

Figuring out what the audience needs to know — and when they need to know it — is one of the trickiest aspects of screenwriting. The novelist can suspend the action for paragraphs or pages to establish background information. Screenwriters can’t. We don’t have an authorial voice to fill in the missing details. Everything we want the audience to know has to be spoken by a character, or better yet visualized in a way that suits the big screen.

So we have to be clever. Sometimes, we use the form to our advantage: A lengthy sequence explaining dinosaur cloning techniques in Michael Crichton’s novel Jurassic Park becomes an animated film strip in David Koepp’s movie adaptation. In most cases, we do more with less, distilling the information down to a minimum effective dose to get the audience through the scene, sequence and story.

The frustration for screenwriters is that many of the decision-makers — directors, producers, studio executives — will have different opinions about that minimum effective dose. Directors will try to cut all the dialogue. Producers will focus on strange details, having read the script so many times that they’ve lost fresh eyes. And studio executives, having faced confused audiences at low-scoring test screenings, will want things over-explained to painful degrees.

But that’s politics. In terms of craft, Sorkin’s point holds: you engage the audience by making them work. One of the best ways is by understanding and controlling what they know.