People sometimes ask me about whether I will write the screenplay for one of my books. The two are very different animals. I know the producer who worked on National Treasure and he sent me the screenplay for it. Screenplays are stripped down, mostly dialog and a little description of action. But in National Treasure, a huge chain reaction car crash is described by using CRASH BANG WHAM in capital letters. That's it. I think it goes on for about 10 minutes on the screen.
I'm not sure I could work like that.
Here's Barry Eisler's takeon it. He shows you a first chapter and then rewrites it as a screenplay.