Simon you are probably right: Add to that the fact my novel writing has slowed to a crawl while I pursue my fascination with learning the art of screenplays, and every thing says I'd be better off using your approach.
However I'm learning how to write screenplays, I'm nowhere near accomplished even when it comes to proper format. It is one way to learn and it is fun to do.
Note: The following is not an argument for the correctness of my position, it is merely an explanation of that position. It also presents ideas that should be thought about by anyone considering doing adaptations, or who is considering making the transition from screenwriting to novel writing or from novel writing to screenwriting.
I learn about getting things on film: Even doing a chapter by chapter basis I have to figure out, "How do I show this visually?" (When I write a chapter knowing I'm going to do a script of it, avoid making the script easier to do. Keep in mind: A novel is a novel, a script is a script: Do what each does best.)
I learn about adaptation: In my novel I may have seven characters performing nine functions. Which functions can I do without: Which characters can I do without: How do I get this reduced set of characters to perform the necessary functions? These are things you do not even think about when writing a novel, yet you must when facing an adaptation of anyone's work let alone your own.
Remember you still must retain the same essence, flavor, texture, of the original work -- Which means you face a set of questions no one else does -- What is the original essence, texture, flavor anyway? When you wrote the original this was the last thing on your mind, now it must be the first.
I also learn about my own novel, about the workings of my own mind, and how the world of the novel differs from the world of the screenplay. One of the things I've noticed about novelists who turn screen writers, aside from Ray Bradbury, is that their novels begin to suffer; they start thinking of story in terms of visuals and subtext, things that add richness to the screen, but makes a novel flat: They start forgetting the rich inner world of the characters which gives the novel its special niche in the world.
Another thing I want to do is to someday write a really good screenplay and then turn it into a novel that satisfies the me who once wrote novels with no idea how to write a screenplay. Most of these adaptations to novel form are flimsy and without real substance no matter how good the movie was: This is because it is easy to slip into the habit of describing what happens in the film without exploring the rich inner area where a true novel dwells.
Once I feel confident in my ability to write actual screenplays I will concentrate on writing treatments, knowing I can turn it into a real script in a reasonable length of time.
