Love this answer by Robet McKee in an interview on Storylink.com.
Debra Eckerling asked him, How important is the process of rewriting?
“It’s absolutely critical. I quote Hemingway in my book who said, “The first draft of anything is shit.” What’s difficult for writers to come to terms with is to recognize that 90% of what we all do, no matter our talent, is not our best work. We are only capable of excellence maybe 10% of the time.
So, how are you going to fill a screenplay with 100% of excellence? Everything has to have been experimented with, improvised, played with, ten times over. Ninety percent of our work must be thrown away in order to ultimately end up with the precious 10% of excellence.
If, for example, you write a 120-page screenplay with 40 to 60 scenes, if you keep every single scene you write, and your so-called rewriting is just paraphrasing and re-paraphrasing dialogue, that’s not rewriting, that’s just polishing. Rewriting means deep structural change in character and story. THAT’S rewriting. If you keep the first draft of your 40 to 60 scenes, you can be sure that, at best, four to six of those scenes are of real quality. The rest is crap. Rewriting doesn’t mean drudgery. Rewriting means re-imagining, recreating, improvising and trying all kinds of crazy ideas. That’s rewriting.”