With the release of Harry Potter & The Order of the Phoenix, I am forced to listen to friends and co-workers and in some cases complete strangers bitch and moan. Indeed, even many reviews for the movie contain the same gripes, and that is that 'it was OK, but they shouldn't have left such-and-such out.' A more common complaint is the simpler, more direct 'it wasn't as good as the book.' This is unfortunate, because a perfectly fine movie is getting short shrift because of how well it stands up to a completely separate entity; the book. Time was when I would be right alongside these people, complaining about how the movie removed my favorite subplot, or didn't capture the essence of the characters as perfectly as I'd hoped. Nowadays I like to think I'm much more enlightened, and oddly enough I owe this all to the Harry Potter series.
I was a bit late on the Harry Potter bandwagon, and Azkaban was the first movie I saw after reading all the books. Strangely, instead of being upset by the (major) omissions from the book to film transition, the film showed me how you could cut out or alter quite a bit and still make a fantastic film. Azkaban is my favorite film in the series so far, and my reasoning, when I try to boil this down to it's essence, is this; The first films were fine, if you wanted to read the book without all those pesky words. The films kept in as many of the subplots as possible, as much of the minutiae that they could manage, but missed a lot of the heart. Prisoner of Azkaban removed as much of the peripheral stuff as possible, but hit the heart dead center.
To me one of the best examples of book to screen adaptation would have to be Silence of the Lambs. Silence of the Lambs should be required reading/viewing in any film class on the subject of adapted works. The movie excises just enough, and fleshes out peripheral characters in order to give voice to what in the novel was internal dialogue. This meant the film wasn't burdened by clumsy narration or even clumsier exposition, but still maintained much of the atmosphere, information and precision of the source material. I'd also give Hannibal honorable mention, and before you stop reading, hear me out. Hannibal was not a very good movie, but keep this in mind; the book was worse. In this case the source material gave them very little to work with, including an underlining psychological explanation for Lecter that's a bit too on-the-nose, and characters that inexplicably act against everything laid down in previous books(keep in mind this was Hannibal Lecter's third outing). The movie trimmed the noxious backstory, and altered the horrid ending, and was the better for it.
I guess the trick is to know what to cut, to be able to discern when the author is going off on an unsatisfactory tangent, or when it's just not necessary to include something. Take American Psycho, for example. Now, I won't say that Bret Easton Ellis is a bad writer, since I did enjoy the novel, in an odd way(and I haven't read anything else, so I can't really judge), but never was there a book more full of things I didn't want to read about. Whether describing sexual acts in a language Penthouse editors would probably blush at, or going on for page after page about where the main character buys his bottled water, the book seemed to urge you to put it down. The movie dropped most of this, and shortened what it did keep. Most likely out of necessity; as boring as it was to read about his choice of neck tie, it would probably not translate to film any better. However, in a true moment of genius, the screenwriter took Patrick Bateman's knack for spouting off about Huey Lewis or Whitney Houston albums and tied those in with the murders committed on screen. This kept the action going, while still hammering home how absolutely empty this man was; less a human being than a collection of appetites.
Of course, inventive editing of the source material is a tricky feat to accomplish. In the Ninth Gate the three screenwriters(including Roman Polanski himself) edited so much out of the book that the movie had almost no plot. The book had several story lines running simultaneously, and more than a few mcguffins. The film pared it down to only one story(and, oddly, not the main one) and removed most of the twists. This wouldn't have been so bad if the film had been any good(actually, I did enjoy it the first time around), but in this case the gap in quality between movie and book is so large that it's hard to ignore. One good thing came of this change; I saw the movie first, and when I read the book I was still surprised by the outcome. Or perhaps I was surprised because I expected the movie ending.
A few other honorable mentions would be Fight Club, which managed to be a rarity; the film that improved substantially on the source material. Mother Night, which accomplished the herculean task of adapting a Vonnegut novel and succeeding(it's perennially in my top 5, and to contrast, check out the horrid Breakfast of Champions film). It is almost a really great movie, despite being a made for TV mini-series. It could be argued that the mini-series is the best place for adaptations, since the expanded time allows for more of the side stories to be left in, and the nature of watching a movie over several days gives you that sense of familiarity and time spent with these characters that a 2 hour movie just can't. The first half of It is a perfect example of this, utilizing commercial breaks as dramatic punctuations, and a template with which to focus on each of the characters in turn. The second half is not quite so good, although it had a lot of stuff to try and fit into 2 hours, and Stephen King has problems with endings anyway. His novels always resolve themselves too neatly, in one notable case literally having the hand of god come down and stop the action.
In the end, we fans are always going to have the book, and if the movie-going audience is content with the often watered down version on the screen, that shouldn't affect us at all. Upset that Quidditch isn't in the Harry Potter movies anymore? Read the book.