Remember: Plot is no more than footprints left in the snow after your characters have run by on their way to incredible destinations.
For me, screenwriting is all about setting characters in motion and as a writer just chasing them. They should tell you what they'll do in any scene you put them in.
The test of any good fiction is that you should care something for the characters; the good to succeed, the bad to fail. The trouble with most fiction is that you want them all to land in hell, together, as quickly as possible.
There is no more interesting place in the world to meet characters than a movie set.
Every human being has hundreds of separate people living under his skin. The talent of a writer is his ability to give them their separate names, identities, personalities and have them relate to other characters living with him.
That was the chief difference between literature and life. In books, the proportion of exceptional to commonplace people is high; in reality, very low.
This list comes from Hal Croasmun of ScreenwritingU. Here's what Hal had to say.