(CNN) -- When people complain about the nearly three-hour length of the first installment of "The Hobbit" film trilogy because the source book is a slim 300 pages, they're forgetting that much of the story isn't from "The Hobbit" itself -- but expanded from the appendices to "The Lord of the Rings."
That extra material is what justifies the longer time on screen, and answers such questions as where does Gandalf go when he disappears? Why is the wizard helping the dwarves on their quest in the first place? What's so bad about dragons? And so, to put a stop to all those moments when you'd otherwise wonder, "But that wasn't in the book!" here are the five major changes from "The Hobbit" the book to "The Hobbit" the films.
The Dwarves' Quest
In the book "The Hobbit," a company of dwarves led by Thorin Oakenshield seek help from Bilbo Baggins because they need a "burglar." That's the term they use, because they seek to steal a whole mess of gold, treasure and other gems such as the Arkenstone from the dragon Smaug. (Or rather, steal it back, since Smaug stole it from them in the first place). In the film, the quest becomes a bit more noble, since the dwarves now phrase it as stealing back their homeland, Erebor, the Lonely Mountain. Even though this only existed in "The Hobbit" as a story Thorin told Bilbo to explain their quest, in "The Hobbit: An Unexpected Journey" film we get to see what a thriving dwarf town looks like and how they lost it, even if Smaug himself is barely glimpsed yet -- a tail here, a wing there, an open eye that looks eerily like the Lidless Eye of Sauron.
"He is an enemy far beyond the power of all the dwarves put together, if they could all be collected again," Gandalf says in "The Hobbit." But when J.R.R. Tolkien wrote the novel he had not yet conceived of Sauron and the One Ring which only came 17 years later in "The Lord of The Rings," so the connection between the Necromancer and Sauron was not made clear. Now that we know they're one and the same, the filmmakers are able to connect the dots for us, by bringing in glimpses of the Ringwraiths (or the graves of the nine men who are the Ringwraiths) and their Morgul blades. And while Gandalf knew that the Necromancer was a great threat, knowing Sauron is no longer sleeping is an even greater one.
Gandalf found Thorin's father Thrain in Dol Guldur, which was near the river where the Ring was lost, as "a prisoner in the dungeons of the Necromancer," he says in "The Hobbit." When asked to explain, he says, "Never you mind." We get much more an explanation in the appendices, which will undoubtedly become a major scene in a later Hobbit film. As a ringbearer, he was hunted down by Sauron's emissaries, who then tortured him and took the Ring from him at Dol Guldur. When Gandalf found him, he was near death, and could barely remember his own son's name. Despite his condition he asked the wizard to give him an inheritance anyway: a map and a key to Erebor. Of course, when Dol Guldur is threatened, Sauron need only move on -- to Mordor.
The Orc Azog
We also will meet another foe, Azog (played by Manu Bennett), an Orc who fights with Thorin's family and during which Thorin earns his nickname "Oakenshield" for the makeshift tree branch he uses in battle against him. Azog is described in the appendices as "a great Orc with a huge iron-clad head, and yet agile and strong." Azog, who killed Thorin's grandfather Thror, is believed to be dead in "The Hobbit," but why kill off a great villain when you can have him hunting down the dwarves throughout the story? His history gives the Orcs more reason to chase down Thorin and company than the other nameless Orcs who would have been required otherwise.
The White Council
In the film, Gandalf leaves the dwarves (and Bilbo) from time to time on their quest -- but where does he go? At least during one of these absences, he consults with a member of his order, Saruman the White (before he goes bad) and two elves, Galadriel and Elrond. It's never specified in the books who are the members of the White Council other than Saruman (who never actually appeared in "The Hobbit" proper).
We do know from the appendices that the council is formed around the same time that Deagol found the Ring, only to be murdered by Smeagol. Coincidence? With the council, Gandalf discusses what kind of a danger the Necromancer really is, why they should destroy Dol Guldur, and why that's necessary for him to help the dwarves -- getting rid of the weapon of mass destruction that is Smaug is their best interest, too, since the dragon would certainly aid the enemy. And why shouldn't they be allies, when both are played by Benedict Cumberbatch?