What a writer does doesn't translate that easily. Had this been a movie just about a writer, I don't think I would have wanted to do it. But the themes of the movie apply to all of us, in that we're all looking backward and forward in our lives, trying to figure out who we want to be, what our purpose is, and how to retain a sense of renewal. A shibboleth in Hollywood holds that good movies are made from bad books and vice versa. So you have to find a way to bring that voice into the screenplay and onto the screen.
Fiction goes inside the characters' heads.
Movies must bring it all to the surface. You have to find other ways to do it in screenwriting. Because it's a wonderful book and he's an extraordinary writer. You want to be inspired by the book, not enslaved to it. You know, I don't have to do all the heavy lifting.