1. It Starts With A Flash-Back
Unlike the beginning of the book - the infamous "In a hole in the ground there lived a hobbit" - An Unexpected Journey begins with Old Bilbo (Ian Holm) relating how Smaug attacked and claimed Erebor for himself, and forced the Durin's folk dwarves into exile. In the text, the history of Smaug and Erebor are related briefly in the Durin's Folk Appendix of the Lord of the Rings, and then in far more depth in "The Quest of Erebor" section of the posthumously released Unfinished Tales. The sequence tells of how the King of Durin's Folk became obsessed with gold, and collected a vast treasure in Erebor, which stood on bountiful seams of precious gems and metals, and how the riches attracted Smaug the dragon, who coveted the treasure for himself. Smaug attacked and drove the dwarves out of the city, killing many in the process. Thorin - the king's grandson appealed to the Elves for help, but were ignored by Thranduil, the Elvenking (and father of Legolas), sparking Thorin's deep-seated hatred of Elves. The prologue format is very similar to the start of the Lord of the Rings, and it transmits a lot of information that wasn't included in The Hobbit text, but which was later embellished by further texts, adding more adult themes to the story, which Jackson has clearly decided to push to the fore-front with his adaptation.