In their book The Tools of Screenwriting, David Howard and Edward Mabley illustrate ten things the screenwriter must accomplish when writing dialogue:
1. It must characterize the speaker, and perhaps the person addressed.
2. It must be idiomatic, maintaining the individuality of the speaker, yet still blend into the style of the screenplay as a whole.
3. It must reflect the speaker’s mood, convey his or her emotion, or provide some window into his or her inner life.
4. It must often reveal the speaker’s motivation or an attempt to hide his or her motivation.
5. It must reflect the relationships of the speaker to the other characters.
6. It must be connective, that is grow out of a preceding speech or action and lead into another.
7. It must advance action.
8. It must sometimes carry information or exposition.
9. Often it must foreshadow what is to come.
10. It must be clear and comprehensible to the audience.