When Thomas Harris wrote Red Dragon and Silence of the Lambs, he shadowed his characters in each scene.
Invisible to them, he took notes on their conversations.
But when creeping into the presence of Dr. Hannibal Lecter for the first time – his skin crawled … as if the doctor knew he was there.
Harris had created a character so REAL, he was acting on freewill.
There’s a muscle fiction writers use to do this: