You really think this is the face of somebody who's lying to you?
Just then, Gandalf appears atop a rock, and shouts that the "dawn will take you all!" He thrusts his staff down, shattering the rock beneath him, and allowing the light of the rising sun through. The trolls are all instantly turned to stone.
The Book: The trolls were arguing about how to cook the dwarves, and after much discussion, Bert came up with the idea to just roast the dwarves now, and eat them later. "No good roasting 'em now, it'd take all night," said a voice, and the trolls began arguing again. They finally agreed to boil the dwarves, but once again, a voice dissented against it, confusing the trolls and making them fight. Each time they came to an agreement, the voice disagreed; each time they heard the voice, the trolls thought one of the others was speaking. Then, a voice that sounded like William's cried out: "Dawn take you all, and be stone to you!" It had been Gandalf speaking the whole time; at that moment, the light came over the hill and turned the trolls to stone.
What difference does it make? This is a pretty huge one. The movie gives significant responsibility to Bilbo here; he very quickly acts after hearing the troll's weakness to sunlight, and acts quite deliberately to stall them before he even knows Gandalf was present. He's the reason the trolls didn't kill the dwarves immediately, before the dawn. Gandalf, too, was given a largely different role (since he was no longer responsible for stalling the trolls); his sundering of the rock was instrumental in exposing the trolls to such sudden sunlight. The conflict is resolved similarly to the book, but with sizable differences that change and expand the roles of the key players.
My Opinion: I'm all for things that make Bilbo Baggins look better, but this one really wasn't necessary. I don't want to start guessing at Peter Jackson's motivations, but, it looks like he's trying to speed up the pace in which Bilbo becomes a useful, contributing member of Thorin's Company. I get that, and I like it, but this is a little too much, too early. The troll incident suffices as an example of Bilbo screwing up royally - it's the sort of thing to show just how much he's grown later. One day, when he's killing spiders and dissing dragons, he can look back on this night and laugh.
"Remember when I nearly got you all killed by trolls? Yeah... good times."
A personal disappointment: I was looking forward to seeing how they were going to make Gandalf's voice sound like the trolls. Was Ian McKellan going to do his best troll impression? Were they going to use special effects to blend the voices together? Were they just going to use the troll's voices, having Gandalf use magic to literally make his voice sound exactly like theirs? It could have been a cool effect. Instead, we got a lame Bridge of Khaza-dum ripoff.