In my review of John Hillcoat's The Road, I didn't really go very far into how the movie differs from Cormac McCarthy's novel, and for the most part the movie sticks very close to McCarthy's template. When Hillcoat does diverge, though, it's noticeable, and I think he strikes a very effective balance between staying true to the source material while making it workable for a different medium. Slavish faithfulness is generally a recipe for disaster in adaptation, and Hillcoat manages to avoid rote translation while still retaining the spirit of McCarthy's writing.
(Spoilers follow.) I read the novel after seeing the movie, so obviously that colored my perceptions, and certainly having read the book beforehand will color others' perceptions in a different way. But overall I come down on the side of the film, if forced to choose, because it presents a more clear-eyed, immediate vision of the stark post-apocalyptic setting. McCarthy famously eschews things like quotation marks and apostrophes, and often doesn't identify which characters are speaking. This sort of impressionistic approach to writing gives the novel a dreamlike quality even when the events are fairly straightforward, and there are times when it's hard to tell whether McCarthy is writing about something that's actually occurring, or a dream, or a character's inner thoughts.
Hillcoat borrows some of McCarthy's dialogue wholesale, and other bits of his prose for use as narration. But just by virtue of seeing the people who are speaking, we're never confused as to who is talking, or whether they're really speaking at all. Likewise, McCarthy's sparse, occasionally obtuse descriptions are replaced by blunt images of devastation, visions that you can't look away from or imagine as somehow more pleasant or mitigated. It's right there, in your face, and yet it's not loud or obtrusive or pushy. It's just the way life is now for these characters, and that's heartbreaking right there.
Hillcoat also makes changes that are more about playing to movie conventions, and I think that's okay, too. He draws out certain sequences for the sake of suspense, but it never feels cheap or unearned. In the book, when the man and the boy encounter the cannibals' house with prisoners in the cellar, they run away immediately and hide in the woods. It's a quick, scary moment, but in the movie it lasts much longer. There's a stronger sense of danger for the main characters as they hide in the same house as these savages, and there's a stronger sense of catharsis, too, when they get away. That's an effect that works better in film, and Hillcoat is smart to use it.
He's smart, also, to expand the flashbacks with Charlize Theron as the boy's mother, which is one thing that a lot of fans of the book were most worried about when first hearing about the movie. The mother is a sort of spectral presence in the book, mentioned periodically, but never quite pinned down. Hillcoat makes her more real, and again it goes to his ability to make this world feel more concrete, more identifiable. There's very little sentiment in the flashbacks, and Theron doesn't play the role for pity. But the colorful, warm looks into the past give an even greater sense of what's been lost in the present.
There's also stuff that Hillcoat takes out, of course, most famously the scene (very brief already in the book) in which the man and boy pass by a camp where people were roasting a dead baby on a spit. That is a seriously fucked-up image, one that strikes you immediately, and to his credit Hillcoat insisted on including it. Then, even more to his credit, he insisted on taking it out when it was clear it didn't work on film. That to me is the sign of a successful adaptation: You are faithful to the letter of the material until that faithfulness hinders your ability to be faithful to its spirit. It's a philosophy that The Road embodies masterfully.