I've become less interested in book to movie adaptations over the years--even though it's become a generic mark of a book's success if it's turned into a movie (though, really, a lot of bad books that everyone quickly forgets have been turned into mediocre movies, while a lot of good books that people still read haven't been turned into movies). You know what movies made from books are like? Fast-forward buttons.
You use a fast-forward button to skip over the things you're not too interested in, bringing in the focus on what you do like. Obviously, nearly all movies made out of books just don't have the time to include everything from the book. So what gets cut out? The things deemed less worthy--or boring. Scenes where nothing happens, scenes with lots of walking, scenes set at a slower pace. Side characters. Minor plot elements. What gets kept, or even put into a more central position? The things deemed to be exciting and visual and well-paced. Action. Fighting. Conflict.
The amount of battle-ness (as a percentage of the whole) in 2005's The Lion, the Witch, and the Wardrobe and then also in Prince Caspian always kind of bothered me. Yes, they're short books, but the battles only take up a couple of pages, which is a much smaller percentage of the whole than what the movie gives them. So why do you have to rush through, say, the time spent with the Beavers (don't let me get into my complaints about how they put them together in the movie) so that you can have time for a long battle? If you don't spend enough time on character, plot, and symbolism, then battles and action become meaningless.
Movies are fast-forward buttons. If a director wants to fast-forward through the bits he thinks are boring to get to the exciting bits, he can. That reminds me of what Peter Jackson said about The Two Towers; he said that he rather liked the idea that Treebeard made people bored or put them to sleep (or roughly the same). You know what's so fantastic about that, as far as adaptations go? That's the director saying, I know this part can be sluggish to get through, but that's the way it's supposed to be and I don't feel insecure enough to be pressured to do it differently. (Because goodness knows, PJ still made time for the long battles.)
If a movie fast-forwards through what the director doesn't like or understand, then no wonder a director must fully appreciate the book in order to make a half-decent adaptation out of it. Because if you fast-forward through the wrong things, what you are left with will be unrecognizable and will become its own, completely separate work. And then what is the point in even basing the movie on that particular book, anyway?