Huge Plot Hole!
Okay let's review: We know Karen Eiffel was writing a novel with Harold Crick as the main character. The voice in Harold's head was Karen and as she wrote it seemed to Harold as she was narrating. In any sense this seemed to be the most logical conclusion. Karen didn't know there was a real person out there exactly like the fictional Harold Crick.
Here's where it get's a little weird and the plot hole comes in: Towards the films conclusion we still hear Karen's voice in Harold's head. Harold found out who the author was and took great lengths to meet her and persuade her not to kill the fictional Harold. We hear Karen saying "Harold remembered a bank of phones in a subway station. "The first one didn't have a dial tone and the second one had some sort of mucus on it. "The third one he dialed and pushed each number very hard" (this isn't word for word but the meaning is the same).
Then Karen new there was a real person named Harold Crick and she shouldn't have been shocked at all when he appeared at her place.
Can anyone explain this?