From everything I've seen and gathered, they are interspersing material from The Hobbit and The Lord of the Rings, including things that are referenced but never actually shown. They are also making heavy use of the Appendices to the Lord of the Rings, which discuss things like the relationship between Aragorn and Arwen in far greater detail than the books themselves do.
It seems, from the Warner Bros. statement, that the plot of the first movie The Hobbit: An Unexpected Journey will mainly follow the plot of the book, but split in three. This should hopefully allow for great attention to detail.
The adventure follows the journey of title character Bilbo Baggins, who is swept into an epic quest to reclaim the lost Dwarf Kingdom of Erebor from the fearsome dragon Smaug. Approached out of the blue by the wizard Gandalf the Grey, Bilbo finds himself joining a company of thirteen dwarves led by the legendary warrior, Thorin Oakenshield. Their journey will take them into the Wild; through treacherous lands swarming with Goblins and Orcs, deadly Wargs and Giant Spiders, Shapeshifters and Sorcerers.
Although their goal lies to the East and the wastelands of the Lonely Mountain first they must escape the goblin tunnels, where Bilbo meets the creature that will change his life forever...Gollum.
Here, alone with Gollum, on the shores of an underground lake, the unassuming Bilbo Baggins not only discovers depths of guile and courage that surprise even him, he also gains possession of Gollum's "precious" ring that holds unexpected and useful qualities ... A simple, gold ring that is tied to the fate of all Middle-earth in ways Bilbo cannot begin to know.
While the second movie was originally conceived as a "bridge" film, that idea appears to have been abandoned. My guess is that the second (and now third) movies will contain much more of the supplementary materials, based on where they decide to split the movies. I'm thinking the first split will occur shortly after Bilbo finds the ring, perhaps around the Mirkwood events, but so far there has been no word on exactly when the splits will occur. Benedict Cumberbatch is providing the voice of the Necromancer, so we will definitely be seeing something of Dol Guldur. This leads me to believe that a lot of things that are only obliquely referenced in the book will be put into the movies (and I'm terribly excited to see this bit, if I'm being honest). Galadriel and Saruman have all been confirmed as well, and that strongly suggests some White Council scenes.
IF YOU DON'T WANT POSSIBLE SPOILERS, DON'T READ THIS PARAGRAPH.
It is rumored, based on comments that Benedict Cumberbatch made earlier this year, that the Necromancer will show up to the Battle of Five Armies in the second/third movie, and the showdown between Gandalf and the White Council and the Necromancer will occur then. This is unconfirmed, but possible given the attempt to integrate tangential plotlines into the story and the attitude PJ has taken of making The Hobbit slightly darker so as to tie in better with The Lord of the Rings.
According to Wikipedia, the studio does not have the rights to either the Silmarillion or the Unfinished Tales, but I wouldn't expect to see events from either of these works anyway; the stories contained within generally fall outside the time frame of The Hobbit movies, which from a cinematic standpoint are intended to function as prequels to The Lord of the Rings films. Supposedly, the third film will end right before the beginning of Fellowship, hence the recasting of Frodo. Indeed, as I suspected initially, the movies are apparently framed around the Red Book of Westmarch and the tale being told to Frodo.*
*I did a bit of peeking around, but this is mainly cobbled together from my memory of all the articles I've read about the movies in the last few years and the Wikipedia article:The Warner Bros release is taken from the article.
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