By lively dialog I don’t mean the characters themselves, I mean the dialogue. There is a difference. Even if you have two slow-witted characters talking in your story (to move the story forward) the dialogue must confound, intrigue or delight your reader/audience.
Go back to your character profiles and, especially for central characters, give them certain key phrases that set a tone for the way they speak. Then during the course of your story you can use the phrases perhaps two times at the most as a touchstone for how that character speaks.
When your characters arrive at a conflict/obstruction/set-back you will know how each one expresses himself.
Here’s a checklist for purposeful dialog in your screenplay.
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. (Use this one sparingly.)
9. Often it must foreshadow what is to come.
10. It must be clear and comprehensible to the audience.
Spend some time out and about listening to people talk. Take notes. Listen especially for phrases, speech patterns, and words to give each of your characters.
Nota Bene: Do not turn characters into “talking heads” at a café telling each other their backstory. Just don’t.
Dialog that is alive with the character personality will deepen reader/audience involvement.