Plus, remember that the average feature-film script is about 120 pages with a lot of white space. It's not like they have to write
War and Peace overnight.
And, yes, the script is often being tinkered with well into post-production. And this is not necessarily a sign of disarray or confusion; it's often just about fine-tuning things as you go along.
"Gee, this scene reads great on the page, but it's falling kinda flat onstage. Maybe we can punch it up somehow . . . or perhaps we should just eliminate it altogether?"
It's funny, actually. People hear about "rewrites" and "reshoots" and such and assume that this is a sign of a troubled production or a "train wreck" in the making, when this often can't be farther from the truth. Making changes throughout is just a routine part of the process.
There's a reason we refer to unfinished movies (and books, for that matter) as
works-in-progress. Nothing is set in stone until you have a final cut or draft--and sometimes not even then!
And changing things throughout is just a routine part of the process. Really.
(Hell, I'm usually revising my books right up until they go to press.)
