I am working on an adaptation of a book into movie format, and I would like to know what experience and guidelines exist for this, so I can improve my way of doing this where possible.
I've tried to do this myself, so I'll pass along what I've learned.
First, note that the audience hears a play about one-third as fast (150-200 words per minute) as they read a novel (500-600 words a minute).
Because of that, a screenplay requires "crisper" writing, with fewer excess words than a novel. A novel might describe a hero's actions as follows: "The hero forced open the door, with rising anger. Then he slammed it shut, hard." In a screenplay, this might read "The hero slammed the door angrily.
The nice thing about a screenplay is that you've engaged the viewer using two senses: sight and sound. Which is one reason you can use fewer words. Whereas in a novel, the reader only "reads."
The biggest different between a novel and a movie is that in a novel, things are described to the reader. The reader can get inside the character's head, be told what the characters are feeling, what the characters and thinking... This doesn't happen in a movie. A movie can only show, not tell. A screenplay has to be entirely visual (and auditory.) This is difficult of course, because you can't say something like "Today, John is upset because his classmates made fun of him yesterday."