Except for the first 100 pages or so, I hadn't read the book when I saw the movie. It was an interesting experiment, and maybe I even enjoyed the movie more as a result. It was coherent enough, for one thing, and I didn't mind the different interpretation of Umbridge because I wasn't firmly fixed on her froglike appearance. Reading the book afterward didn't seem spoiled, because there are always things in the book that don't get into the movies. Apart from this, all the movies I've seen or will see follow my reading of the particular book, and up to now, I've thought that without the books I wouldn't have 'gotten' the movie. Now I'm not so sure.

I wondered some if Rowling wrote passages thinking they'd be perfect for some great special effects. The first couple such I saw weren't even alluded to in the movie.