Avoid Exposition

Exposition, for those unfamiliar with the term, is information presented in order to explain story elements. A perfect example is the long speech given at the end of a murder-mystery, in which the detective reveals who the killer is, and how the dastardly crime was accomplished. In few other genres can a writer get away with such a pedantic approach. Having your characters explain the story, justify or clarify their actions, usually means that the story wasn't clear enough to begin with. Explaining motivations after the fact through exposition undermines dramatic tension. The most effective way to reveal the essential information is gradually, and in the context of events.

In suspense stories, a genre in which many details may be deliberately obscured, important information is revealed in stages. In "North by Northwest," the story's protagonist is chased by persons whose motivations are unknown. Yet the protagonist's motivation is clear - stay alive. Bit by bit, the real story is revealed, but in a manner that avoids long-winded explanations.

If you have clearly presented all of the necessary elements, the audience will put the pieces of the puzzle together to complete the overall picture.

Hear the Words

One last and very important point, is that dialogue on the page is very different from dialogue as spoken in someone's mouth.

Screenwriters often speak their dialogue aloud. There exists no better way to hear its rhythms, taste the flow of the language, and connect with your characters' voices. Play the parts to yourself, and listen to what your creations sound like. Your characters are a part of yourself. Talking to oneself may seem odd, but holding out-loud conversations between your characters is one of the most effective ways to get to know them. Even if you aren't inclined to act on-stage or in front of a camera, if you learn to take an actor's perspective when creating your characters, it will give you greater insight into their minds and hearts.

Related Links:

Editing Dialogue
A great collection of pages on solving common dialogue problems, from Richard Toscan's Playwriting Seminars.

Subtext
Richard Toscan discusses the subtleties of subtext.