"Fucking elves. You know they hug trees, right? What do you think they do with small shafts of wood?"
The Book: No such scene occurs. There are no orcs hunting Thorin's Company.
What difference does it make? The inclusion of the orc pack is huge. Not just in this scene, but in the movie itself. Unlike the trolls, or the Goblin King, the orcs are not present in the book in any way. Peter Jackson has added a new threat that constantly hounds the dwarves. This means that not only are scenes like this one invented out of whole cloth, but they have to be squeezed in between scenes that actually did happen in the books. This necessarily throws off the pacing of The Hobbit, and changes the order and reasoning of events. Furthermore, by making this a recurring threat, it adds an entire new plot line that requires its own climax and resolution. The more changes like this that occur, the less faithful the movie is to the book.
My Opinion: I'm of two minds here. First, it's a change that didn't have to be made. Unlike the fight with the trolls, or some small condensing of dialogue, this is not a change that was made to streamline the book into a good movie adaptation - it's a wholly unnecessary addition. It was exciting to watch, certainly, but that doesn't justify adding a villain just to chase the characters from one book scene to another. This is more than a mere modification. At this point, Peter Jackson is changing the story of the book. But on the other hand, he did need a reason to get Thorin to go to Rivendell. Thorin's racism, which is in my opinion an excellent bit of characterization, is not in the novel, either. He wouldn't have just taken the Company to Rivendell unless he absolutely had to. Furthermore, since the movie version of The Hobbit is divided into a trilogy, each movie would need a small story arc of its own. A buildup in the first movie for a climax in the second or third would just be bad storytelling. At least Peter Jackson put the orc attacks in places where they would not conflict with the rest of the novel's story. It's better to use it to propel the dwarves to Rivendell than, say, to have the orcs be allies of the trolls or something. It's not a change that I'm really happy with, but it's one that I can understand, and appreciate, the reasoning behind.
That said, I want to talk about what the fuck Radagast was doing. He said he would lead the orcs away, and that's what it looks... at first. He bragged about the speed of his rabbits, and sure enough, he's faster than the Gundabad Wargs. The movie shows Radagast exiting the forest first, then chased by the orcs, then Thorin's Company running out on foot. So why the hell did the dwarves keep running into the orcs? Why were they anywhere near them when Radagast just speeds away? Was he just going in circles or something? Did he forget what he was supposed to have been doing?
To be fair, we did watch him get stoned just five minutes ago.