So, before Duff can steal anymore of my thunder (see the comments), I'll continue.
As I ended my previous post, "My thoughtful answer comes back to the question I asked earlier: why bother to take a good book and make it into a movie?" I agree with Duff: despite my somewhat reactionary approach to movie adaptations of books, I don't actually want to see a movie that's an exact replication of the source book. (Sin City is redeemed, in my mind, because it did some revolutionary stuff within the visual medium of film.)
If a book is already really great as it is, you'd better have a good reason for wanting to recreate it. Wanting to cash in on something's existent popularity is not such a reason, but I do believe valid reasons exist. Duff makes a really good point about the retelling of stories, and a particular retelling being expressive of how that story came across to the individual doing the retelling. Even wanting to make a great story more accessible isn't necessarily harmful; I do worry that today's children, supplied as they are with myriad mediocre retellings of my favorites books from childhood, will not read the actual books, but isn't also possible that a good version of something will result in the viewer seeking out other versions, including the original source?
The books doesn't even need to be the ultimate authority. I'm certain there are movies that are better than the books they're based on, for whatever reason.
Good stories have been retold for generations. They've been modified, updated, added to, given new perspectives, changed shape, and thrived. Film is just one outlet for this process of adaption and adaptation. And if you're going to choose film, I want to be able to recognize why.
What I look for in movie adaptations, besides good storytelling, is for the reason that it had to be retold through cinema. If you're making a book into a movie, I want to see something done with the story that only film can do (just the mere fact of wider potential audiences isn't enough here). The most obvious difference between books and movies is the visual element (I almost feel stupid, pointing this out): movies show what you would otherwise have to imagine. It's definitely not bad to get to imagine these things for oneself, but it can be fun to see how someone else interpreted it. Fantasy and sci-fi features stand out in my mind; for example, I thought the Harry Potter movies did a really excellent job of visualizing the magical elements (whatever the die-hard bookfans might say about the films' corruption of plot). I haven't actually seen A Scanner Darkly yet (though I did read the book in preparation for the movie), but I was excited to see what the anonimity suits (and from what I saw in the previews, they had a pretty cool idea).
Next most obvious difference is the addition of a soundtrack; not only do we see the world, but we can hear it, too, not to mention scores and pop songs. The use of sound and music add layers to any film and has a huge impact on mood and atmosphere. If used really well, I believe music can convey in a film what a narrator (perhaps not included in the adaptation) does in a book. Relatedly, in the beginning of the novel Nightwatch, a characters has song stuck in his head; I don't remember if the movie used the song or not, but that would have been really interesting to see (or perhaps I should say "hear").
Film also has an aspect of temporality to it that doesn't necessarily exist in written prose. Scenes can be sped up so that you get a sense of panic, and can't catch everything going on; they can be slowed down to indicated tiredness or boredom. Prose can certainly play with timelines (look at Slaughterhouse 5, though I won't comment on how the movie worked in this respect as its been a long time since I've seen it), but because of the visual element a flashback in a film can be used with great force.
Finally, as Duff also pointed out, there is just the quality of the movie itself. No matter where it came from, the film should be able to stand on its own. So what if Jumanji differed so drastically from its source material? If the spin-off ideas had been good ones, the movie could have been good anyway (and some people felt that it was). From what I've read, I think Night Watch and Day Watch fall into this category; comparing books to movies, the two movies are both based on the first book in the series; relationships of characters to one another are changed; not all of the events are included and priority of certain events might be shifted. Maybe I'll feel differently after reading the novels (a project in progress right now), but I really enjoyed the movies on their own, and I hope that after I've read the books, I'll appreciate both experiences in their own rights.
So, those are just some of my thoughts on books made into movies; they are only some of my thoughts on the subject, so feel free to share your own ideas!
Speaking of books made into movies, the two most on my mind upcoming are Stardust and Coraline, both based on books by Neil Gaiman. Stardust is out in August and I cannot wait; I might reread the book in preparation, but I also might not.
Coraline (directed and screen written by Henry Selick of The Nightmare Before Christmas, with a soundtrack by They Might be Giants!) is due out for Halloween 2008. From Neil Gaiman's blog, five years ago he had this to say about the screenplay:
Henry's first draft of the script was utterly faithful to the text of the book -- if anything, too faithful. This version was both looser and truer to the spirit of the book -- he'd added a character, made the beats in the first act slightly different, but the changes were the all kind of changes that need to exist when translating a book into a film, and the core characters -- Coraline, her parents, the Cat, the Other Mother -- and the story are still just the same.So, that's pretty interesting, given this discussion!
By the way, I will probably get to see Neil Gaiman in person next month, at the San Diego Comic Con! which is pretty exciting.