I think the filmmakers were smart to decide that they were going to actually adapt these books into films rather than be beholden to them in a slavish way. Anyone can disagree with individual changes they might have made, but the idea up front to give themselves permission to change almost anything, was a smart. The "film story" had to flow well, aside form what was in the books. Imagine if Frodo gets the ring in the film and waits sixteen years before decideing to go on the quest. That would have killed the film's narrative.
However, once they came up with their outline of how the events would play out as a film, they went back and inserted as many details of Tolkien's story as they could, right down to Bilbo's brass buttons.
One of my favorite bits of writing in the film is what Gandalf says to Saruman about the Palanteer. He covers it up and says "we don't know who else may be watching." That line is a brialliant bit of script writing. With it, we get a sense of how the Palantir's work, that if another wizard has one, he can see what Saruman might be seeing. We get a sense that they are all conencted. We are saved from real exposition about how they might actually work, yet we are left intrigued to the possibilities.
That's how you adapt a novel to a film: you cut everything you can out, but you treat it like it was there, and make its absence interesting and intriguing.
) to escape the sewers and find their way back.. i went 
(Though the fact that she made me a fighter pilot is ironic, since I don't even have a driver's license. I don't know if she knew that, though.)
