(OMG SPOILERS AHEAD RUN!!!)
The premise of the narrative is that the protagonist, Harold Crick, one day begins hearing the voice of author Kay Eiffel in his head, narrating his life. He eventually figures out that he is a character in one of her books, and since she kills off all her characters, he must find a way to stop her from finishing her manuscript.
In the end, all parties, including Harold, decide that Eiffel's book is just so good that it needs to be finished, and therefore Harold must die. This is, of course, absurd, but I can suspend (just barely) my disbelief and imagine that the book was so moving Harold and everyone else found its completion more valuable than his mundane life.
The problem with the narrative is that Harold and Eiffel meet, and Eiffel becomes aware of what she is doing. At that point the conceit of the story ceases to be fantastic and becomes mechanistic.
In The Fantastic: A Structural Approach to a Literary Genre, Tzvetan Todorov defines the fantastic:
In a world which is indeed our world, the one we know, a world without devils, sylphides, or vampires, there occurs and event which cannot be explained by the laws of this same familiar world. The person who experiences this event must opt for one of two possible solutions: either he is the victim of an illusion of the senses, a product of the imagination – and the laws of the world remain what they are; or else the event has indeed taken place, it is an integral part of reality – but then this reality is controlled by laws unknown to us. Either the devil is an illusion, an imaginary being; or else he really exists, precisely like other living beings – with this reservation, that we encounter him infrequently.
The fantastic occupies the duration of this uncertainty. Once we choose one answer or the other, we leave the fantastic for a neighboring genre, the uncanny or the marvelous. The fantastic is a hesitation experienced by a person who knows only the laws of nature, confronting an apparently supernatural event. (25)
Once Harold meets Eiffel and they confirm to each other that they are each real, the phenomenon becomes a real phenomenon, and like all real phenomenon it is subject to rules, even if we don't understand those rules yet.
In other words, at this point in the narrative the phenomenon shifts from something fantastic happening to Harold to a super power that Eiffel possesses. As every comic book reader knows, super powers follow rules and have loopholes. And the film itself establishes some rules for her power: things only happen to Harold once Eiffel has typed them on her typewriter; writing with a pen won't do it. And they only fully take effect once she has typed the period at the end of the sentence.
So, once we've established that the phenomenon is real and follows rules, any sensible person would try to find out what the rules are and game them. Rather than choose between Harold's life and the greatest piece of literature of the decade, why not try to game the system so you can have both? Anyone faced with this dilemma would have tried to, for instance, change the main character's name to see if that would break the link between the manuscript and Harold. Or, since pen writing doesn't count, have someone else try typing her penned manuscript, to see if Eiffel personally typing is required for the power to be activated. Or she could write a happy ending, then throw out the last few pages and write the ending she wanted; presumably her power doesn't extend to changing the past.
Obviously, a story about Eiffel testing the rules and limits of her newfound super power is not the movie the writers wanted to make. But the fact that the characters don't act in a way that any sensible person would without any explanation is a glaring problem with the narrative.