get into (one's) head
1. To begin to understand how one thinks. You need to get into the criminal's way in order to find a way to stop him.
2. To intentionally disrupt or undermine one's focus, confidence, or performance. The opposing coach called a time out at the last second to try to get into the kicker's head.
get into one's head
Also, get or take it into one's head . Form an impression, idea, or plan. For example, What strange idea has she got into her head? or He took it into his head that you want to quit. [Late 1600s] Also see get through one's head.
get into someone's head
If you get into someone's head, you understand what they are really thinking or feeling. The author has really tried to get into the heads of these criminals.