In Stranger Than Fiction, Karen Eiffel is an author who unknowingly is writing something that is actually happening in the real world. The protagonist from her book, Harold Crick, is a living, breathing human being. The question, however, is whether Miss Eiffel is the author of Harold's life, or a commentator. This question is only an implied question, and is never explicitly answered. It seems that she has the power to control his fate, as seen in this clip.
Even this, however, does not necessarily imply that she authors his life. Even though everybody in the story believes that she does.
It reminds me of a series of books that I read called Myst. You may be familiar with the computer game of the same name. These books were written by the creators of the game and take place in the same world. If you are unfamiliar, the only thing that you really need to know for this purpose is that there are books that link you to other worlds, like this:
Myst Linking Book
In the series, there are two characters that you need to know about: Atrus, the protagonist, and his father, Ghen. They are the people who write the Linking books. Ghen believes that he is a type of god that creates worlds by writing words on paper. Atrus, however, believes that the books that they write are simply linked to preexisting worlds. I can't remember if there is a definitive answer, but if not, then it is strongly implied that Atrus's belief is correct.
Before I tell you what I think, let me first lay it out on the table that I am a firm believer that God gives us free will. I do not believe that God is a micromanager. It just does not make sense to me that God would create the cosmos, and then decide every little happening that goes on. God wants us to love him because we want to, not because he makes us.
Not God.
Having said that, I personally believe, like Atrus probably would, that Karen Eiffel is writing a book that narrates the life of a preexisting person. The words she writes are determined by Harold's actions and circumstances, not the other way around.