I do agree that screen-writing classes are essential for the one wishing to learn the craft, but, suppose, it is impossible due to financial reasons or just because there is no decent film school in around 400 km from your native town (in fact, there is not a single one at all, in my case). Then the only choice is reading scripts and books on the screen-writing. Such authors as Syd Field, for instance, give the theory in a very clear and precise manner. If you just care to switch on, then in a very limited period of time you will see the inciting incidents, dramatic needs, acts divisions etc. when reading scripts. Again, I'd like to stress, that proper education, supervised by an experienced professor, is the best way to master the craft.
Anyway, I sure would like to have the printed script of Inception. But I'm almost sure that there will be none on sale in my country. So, I'll just hope someone scans the book.