When I was 30 or so - by that time I had become an assistant D.A. - I decided I would try to write a novel. To be clear: I did not decide to become a novelist. Honestly, it never crossed my mind that I could actually earn a living as a professional novelist.
If you get a book which is 600 pages, you have to reduce it to a script of 100 pages. In two hours of film, you cannot possibly include all the characters.

