Warren Ellis is writing a screenplay for a new videogame, and has a production blog about his experience.  This post On Writing a Screenplay outlines a process that I’ve been meaning to try for my next story:

There are probably two different kinds of screenwriters. The kind who start with a blank sheet of paper and just bull through to the end of the thing, and the expanders, like me.

People like me work as if they’re applying coats of paint to the same surface. The only blank sheet of paper is the original premise document. When it comes to writing the outline, I paste the original premise document into a new file and start writing over the top of it: expanding it, rewording it, moving scenes around. And when it comes to the screenplay, I paste the outline into a new document, and start writing over the top of it.

http://castlevaniadraculascurse.com/