"Dude, what the fuck? This is my house!"
Gandalf explains to the nonplussed dwarves that the giant bear was, in fact, their host - a skin-changer named Beorn. Gandalf tells everybody to get some sleep, and they bed down in the long hall. Beorn is later seen keeping watch, guarding his home against the orcs.
The Book: Bilbo asks Gandalf why the large rock the eagles landed on is called the Carrock. Gandalf replies that it is called the Carrock because "a very great person" calls it that. The wizard explains that this person is a skin-changer named Beorn, a man who can be kind, but is terrible when angry - and he gets angry easily. Therefore, to keep him in good humors, Gandalf tells the dwarves not to come to Beorn's long hall all at once. Instead, they ought to wait behind, while Gandalf approaches with Bilbo, alone. Then the dwarves are to arrive in pairs, once every five minutes. Gandalf works this into a conversation with Beorn very cleverly, so that Beorn isn't bombarded with a dozen dwarves all at once. This keeps the skin-changer interested, and in good enough humors to invite everybody into his hall for dinner.
My Opinion: I didn't care for this change, but I can understand why it was made. Watching two dwarves introduce themselves at a time to their begrudging host is something that's much easier to read than it is to watch - especially to filmgoers who just saw that exact thing happen in the beginning of the first movie. Even that didn't play out as slowly as it did the book. But did Gandalf's clever scheme have to be a thrilling chase? Just because perfect fidelity might not have been an option, it doesn't have to turn into some kind of theme park thrill ride.
And this isn't exactly a comparison, but I just gotta ask - what is up with all the sudden, drastic changes in scenery? I understand trying to make the whole thing feel epic, but this is not the first time that the movie has drastically switched from one kind of locale to another in one scene. First they're on a mountain, then this vast prairie, then they're in the woods. It's jarring.
You could cut out all these scenes from the movie and shave off, like, 45 minutes, easy.