Benjamin writes in with another question:
"Is it advisable to write a scriptment rather than a script? Like the one James Cameron wrote [for Avatar]?"
For those not in the know, a "scriptment" is a hybrid of a script and treatment, combining elements of both. It's longer than a treatment - largely because it includes and summarizes dialogue.
In a word: no. At least not if you harbor any delusions about selling the scriptment before writing the full script. Unless you are an established, known quantity in this town, no one is going to pay you money for a script unless you've actually written a script. Guys like Cameron can use scriptments because they're not only established writers, but also the director who'll be bringing the project to life. Thus, the scriptment is a tool that they can show around to their producers, studio execs, or agents saying, "This is is the story scene-for-scene, beat-for-beat. You know what I can do, so if this piques your interest then maybe we can be in business together."
It's useful because there's a lot more detail to a scriptment than there is to a pitch or even a treatment. Making these details less vague is especially useful when you're talking about a $100+ million dollar film.
I personally have seen a few scriptments for franchise films done by companies I've worked for. In those cases, the known quantities were not only the writer/director, but also the characters and the world they inhabited. These sorts of scriptments not only gives everyone a strong idea of how the story will continue to evolve, but it also could be of use to the various departments in preproduction. When the studio has already selected an impossibly close release date, time is of the essence.
And yes, a scriptment might be useful to you as a document for your own personal reference. I can see the value in laying everything out scene-by-scene and then summarizing each scene as a way of revving up for fully writing every scene. (Personally, I can't work this way because I feel it robs my ideas of their freshness. I'll work on structure for a while, and refine my treatments over weeks or months, but when working on the first draft, I don't like to overthink every single conversational beat too long before I actually start dialoging.)
But for an unsold writer, selling a spec script would probably be a piece of cake compared to selling a scriptment.