Here's a fascinating article by George Jonas, the man who wrote Vengeance the book that was the basis for Steven Spielberg's Munich (2005).
Jonas describes in detail how movies are made in Hollywood -- from book to screenplay, to screenplay, to screenplay, to scree... Bottom line: no matter how good is the script, it is re-written until it conveys what the director wants it to convey.
It's worth a read if only to learn how to write a screenplay for Hollywood.
>> Read the George Jonas piece in Macleans
Note: The original Macleans article requires a paid subscription. The link is to a re-print of the article on Usenet.