Whats it called when someone believes they are someone else?!

I'm going to open up for the first time about how I get with my social worker tomorrow morning. What I mean is that when I see someone or something I become it. For example I saw the show dexter and then believed I was this intelligent killer no one knew about and began acting as confident as him and even my voice when I would think sounded like him. Ive never told anyone about the way I get as it has happened tons of times with different things. Sorry for the repost I'm just looking for answers to have a heads up tomorrow.