There's something to the idea of the autonomous character. Big chunks of our wetware are devoted to simulating other people, trying to figure out if we are likely to fight or fondle them. It's unsurprising that when you ask your brain to model some other person, it rises to the task. But that's exactly what happens to a reader when you hand your book over to him: he simulates your characters in his head, trying to interpret that character's actions through his own lens.

In Praise of Fanfic. Locus (May 2007). Retrieved on 2007-05-21.