Movies and books have to be very created in very different ways. And I don't mean the obvious. I mean the way the story is portrayed and put together. It's most obvious when they turn a book into a movie (and sometimes the reverse). The bonus features on the first Narnia movie talk about some of their decisions on turning a book into a movie.
The pacing has to be shifted to fit the medium. They'll rearranged some scenes to maximize impact. For example, the scene where Aslan returns was shifted back until after the start of the battle. So when Peter leads the Narnians to war, you don't know that Aslan will be coming to their aid (also, I had this suspicion which was confirmed when I watched the bonus features - the White Witch takes the mane she cut off of Aslan and wears it as a collar during the battle).
Inevitably the filmmakers have to leave some scenes out, for timing. And sometimes they'll add scenes to make certain things clearer. In this case, they added the entire battle scene that was only mentioned briefly in the book. And they did a great job (which I can't say of the second Narnia movie). The battle shows Peter and Edmund fighting and earning the right to lead this kingdom.
They often add little visual details too. For example, the White Witch's ice crown slowly melts throughout the film.
They also don't tell you that Aslan is a lion. If you're not familiar with the books, you wouldn't know until he first appears. Which gives his introduction more impact.
And despite all of the differences between visual and written media, I still love to study movies and shows for ideas. It will never be the same, but there is often some sort of inspiration!
