When did you realize...
...that the narrator was also Tyler Durden?
I read the book a long time after seeing the movie so I knew all along, obviously.
As I was reading, I was curious as to when it would've dawned on me that he had a split personality.
In the movie it was during the climax and it was a big "Holy Shit!" moment. But in the book it seems like you could've figured it out earlier.