I am gonna assume you haven't read/watched Monster. That's what Lunge does. He talks to himself, repeatedly saying that he is some person, the killer, he convinces himself of that, says what he is doing and upon entering that state he is capable of ascertaining and re-enacting the killer's state of mind, motives, method and can even predict their next moves.
If you want to see it in action check this video. It should be timestamped, but if not go to 3:30 and watch until 6:00.