In fact some of what happens, mostly the stuff to do with gandalf, is not adaptation but story originally from Tolkien. The book The Hobbit focuses on events entirely from Bilbo's perspective, but he did write other things for the time during which the Hobbit took place, one of which was a brief story about the events surrounding the Necromancer (Sauron) and his rise and fall.

Radagasts involvement was not originally in that story, but I think they did a good job putting him in, and at the very least it gave Gandalf someone to interact with.