I wrote my first screenplay from scratch--no outline, no synpsis, no treatemnt, just broke open my Final Draft and started writing. It was a 32 page thriller. I trimmed it down to 28 pages on my rewrite and that was it.
I have since learned the old-fashioned method of writing a script where you get the idea, THEN you sketch out a rough synopsis, THEN you make a step outline, THEN you write a treatment, THEN you write the scirpt. At first I balked at all that, thinking it was just a lot of needless formality. But then I found I was inadvertantly doing all of the above anyhow, and that I was doing it in that exact order. So the whole process turned out to be quite organic.
As for my own treatments, I usually start out writing them JUST for myself, and JUST as a tool to get to the actual script phase. But my treatments are a bit unconventional in that I tend to cram them with loads of dialogue. My scripts are usually very dialogue-driven, so it just comes out that way. When my pre-script treatment is done, I find it often exceeds 50 pages, and is very very dialogue-heavy.
Now, after I'm done with the script, I pass the script around to various friends for review, but not all of these friends are script-savvy. So, to appease the script-challenged, I go back to the treatment and re-work it into a literal short-story with formal dialogue and a distinct narrative. These treatments (or "scriptments") can easily exceed 100 pages. And my non-scripty friends like reading those.
Some of you might think that this overly long post-script treatment is kinda silly and alot of extra work for nothing. But I find that this re-working of the treatments can not only broaden the number of people who can read my story, but it also helps me spot plot details I would have otherwise missed. And the REALLY cool thing is, in the event I actually land a sale, I now have the novelization already finished.