Before you write, be sure you have a written agreement. If the author is paying you to create the screenplay, you are not the copyright holder by considered an "employee". The written agreement should state what money and credit you will receive as part of the adaptation before getting too far along. I know I may sound rather mercenary but without that in place, you are guaranteed neither money nor credit if it is sold.
Realize that writing a novel is different from a screenplay. A novel needs to be endlessly detailed, can take the reader inside the characters' heads, and can run on many pages. A script is a production guide that provides sufficient details to tell the story and help other creative people (actor, director, art director, dp, etc.) re-create the story. The story needs to be structured and paced to be complete in 120 pages. A movie is primarily visual with auditory highlights. Inner thoughts need to be externalized.
That being said, not every novel makes a good movie. Not every novelist will be pleased with the needed deletions to adapt it to the screen. So when you work with the author, you may need to rein in their desire to include features that actually slow the pace of the film.
1. When adapting, always start with a summary. The story summary should be no more than twelve pages. Others have different approaches, but for me, that will force you to capture the key story elements. This should be a "Cliff Notes" version of the novel without dialogue.
2. Next lay out the basic structure. You can use Michael Hauge's Six Stages, Blake Snyder's formulation, or whatever format that you feel comfortable with that provides:
a. Introduction (key characters and the problem)
b. Development (solution of problem, struggle of characters, new problem)
c. Main Action (dynamics of characters, growth, and tackling new problem)
d. Climax and Resolution
It helps to have this in your mind when you go back to adapt the story to a screenplay format.
3. Now edit the summary so it fits the basic structure. This may mean deleting or even changing elements from the original story. In the latter case, it is good to have the author understand that you will need some creative license in adapting his/her work. Remember to give the author first credit.
4. With the revised summary, now treat each paragraph as a scene. It may need more development or less. Don't rush to put in dialogue. Since a film is principally visual, the script's story needs to be visually evident without words. It's a good acid test.
5. After the scenes and action are in place, now go back and add dialogue. Be sure it is tight and relevant. If a character doesn't need to speak, then don't put words in their mouth.
A few "Don't"s
1. DON'T try to translate every chapter. You want to capture the gist of the book, not be literal.
2. DON'T try to mathematically interpolate based on pages. The novel has 240 pages, the script 120 so every script page should cover two novel pages. BAD LOGIC!
3. DON'T include every subplot! Not every subplot is integral to the overall story. You must be willing to throw out a lot of interesting material in the interest of advancing the main story. Authors often struggle with this having been so close to their works.
4. DON'T adapt an author's work without their (or their publisher's) written permission! That's a violation of copyright.
Good luck!
