Pericles said:
Like Titan mentioned above, I began reading the books as a result of watching the movies. I tried reading them when I was a teenager, but I just could not grasp Tolkien's writing style and did not finish the Fellowship. I am almost read through the trilogy now, and what strikes me as a strong deviation from the books to the films is the introduction of romance between Aragorn and Arwen. I was expecting it to play a prominent role in the novels, but it appears that it was something that Jackson developed mostly on his own, as there is just an occassional mention of the relationship in the novels. This may change as I finish TRotK, but I imagine Jackson was inclined to include the relationship to pull-in a wider audience.
My greatest fear concerning The Hobbit is that it will not live up to the lofty expectations set before it by what is THE definitive resource on the topic; the Rankin/Bass animated film. 
(just kidding)
Most of the Aragorn/Arwyn stuff is from the Tolkien. It's in the appendices at the back of Return of the King.
)
