J. and I took a free 2-hour screenwriting class given by the Austin Film Society. The woman who teaches the class offers a much longer class where students work on original scripts over several weeks -- this was just sort of an introduction and sales pitch for her particular method.
She takes the traditional 3-act structure and breaks if down further into 8 sequences. She also stresses specific relationships among the protagonist's flaw, the first act "event," and the nature of the crisis at the end of the second act. It was interesting. I actually came home and made a couple small changes in the script I wrote last year, based on her method. And it gave me some things to think about in the script I'm drafting right now.
Experienced screenwriters will tell you that every screenplay fits into a very neat structure based on Aristotle's Poetics. And it's pretty much true.
I go back and forth in my opinion about where in the process it's useful to start thinking about this theory. It's useful to have something to hang the story on. Strict rules can stimulate the imagination, like in poetry. On the other hand, when it's early in the process, expecting elements of the story to conform to specific narrative demands may have a way of shutting down the imagination. I suspect, when I'm watching a movie and the story feels forced, it's because the writer pushed the formula onto the story too hard, too early.
But it's hard to argue with something proven to work for over two millenia.