"A forthcoming journal article in Psychological Science reports on the research of scientists from the Dynamic Cognition Laboratory at Washington University in St. Louis into what brain activity takes place while we read narrative stories. The study concludes that our brains simulate the action in the story, echoing it as we read.
I've always assumed that this was the case -- especially when it comes to character motivations. When I hear the voice of a loved one in my head, cheering me on or disapproving, I know that this is my mental simulation of that person. When a character does something in a story and I feel for him, it's the same kind of simulation. And when I try to write a character doing something "wrong," I know that this, too, is part of the simulation, and the resistance I feel there is the same as the resistance I'd feel if I tried to imagine my mother committing an ax-murder."