If a novelist has created vivid characters, interesting relationships, settings the reader can easily imagine, and intriguing stories, a screenwriter has loads to work with. The challenge comes with deciding what to cut and what to keep.
All I can guess is that when I write, I forget that it's not real. I'm living the story, and I think people can read that sincerity about the characters. They are real to me while I'm writing them, and I think that makes them real to the readers as well.
One of the things I've found most challenging about writing a multibook series is keeping it fresh and evolving while still delivering the familiarity that keeps longtime fans devoted to the characters and story world.