How does Bilbo manage to sneak up on the trolls? Why does Bilbo decide to try to pick the trolls pockets?

1) But at any rate hobbits can move quietly in woods, absolutely quietly. They take a pride in it, and Bilbo had sniffed more than once at what he called “all this dwarvish racket,” as they went along, though I don’t suppose you or I would have noticed anything at all on a windy night, not if the whole cavalcade had passed two feet off. As for Bilbo walking primly towards the red light, I don’t suppose even a weasel would have stirred a whisker at it. So, naturally, he got right up to the fire—for fire it was—without disturbing anyone.

2) . He was very much alarmed, as well as disgusted; he wished himself a hundred miles away, and yet—and yet somehow he could not go straight back to Thorin and Company emptyhanded. So he stood and hesitated in the shadows. Of the various burglarious proceedings he had heard of picking the trolls’ pockets seemed the least difficult, so at last he crept behind a tree just behind William. Bert and Tom went off to the barrel. William was having another drink. Then Bilbo plucked up courage and put his little hand in William’s enormous pocket. There was a purse in it, as big as a bag to Bilbo. “Ha!” thought he, warming to his new work as he lifted it carefully out, “this is a beginning!”