Mark Mills who writes mainstream novels. He quotes scriptwriters to say that "character is action." The characters have to hold the reader's attention as much or more than the plot. Very true. I find it hard to read books where I don't care about the people. I don't have to like all of them, but I do feel a need for somebody to root for.
Mill's advice to new writers is to not launch into a story until they have a clear idea of the world and the characters they intend to create. As he says, "only start writing when you are ready to." I find that interesting because it's so incredibly true for speculative fiction writers, where the world they create is not necessarily similar to the one we all live in.
When asked if there was a secret to writing, Mills suggests writers write the kind of story they want to read and hope that enough people agree with them. An interesting point. If TV shows are any indication, my taste does not match that of the American public.