Well, with book 5 coming out this spring, she's only got 2 more to finish, and then she's done... In it's originals inseption, Harry Potter was meant to be 5 books, and JKR has said when they're done, she's likley never gonna write about him again...

And well, as far as Robbie as Hagrid goes, that's who she was thinking of when she wrote the character of Hagrid, so for Robbie to have not played it, would have been silly...

"the people who wrote the screenplay will surely not have been told secrets about the future of the story"

You don't think that JKR is up to her eyes in the screen writing of these movies?