In theatre, there’s something called a prompt book. The prompt book is what the stage manager has, usually a loose-leaf book with all the lighting cues. I make a prompt book out of the novel. In other words, I break the novel, and I glue the pages in a loose-leaf, usually with the square cutout so I can see both sides.
I have that big book with the notes I took, and then I go and I put lots more observations and notes. Then I begin to go through that and summarize the part that I thought was useful. And quite naturally you’ll see that the parts fall away, or that you have too many characters, so you know that you have to eliminate some or combine some. Working on it this way, from the outside in, being more specific as to what you think… then when you finish that, you are qualified perhaps to try to write a draft based on that notebook.
In the case of “The Godfather” I did that, and although I had a screenplay, I never used it. I always used to take that big notebook around with me, and I made the movie from that notebook.