Why do I start acting like people I hang out with?

I've always sort of known I do this, but recently its become extremely clear and obvious. I'll begin to mimic certain personality traits, quirks like how someone laughs or talks, or certain subtle movements like if I hang with a friend who flips his hair alot then I'll eventually start doing it. Its not just with people I hang out with either. When I watch movies I sort of take on the vibe of the movie. For example I was recently watching Donnie Darkko (great movie I highly reccomend it) and I started to think of really weird unusual things that where kind of related to the dark, creepy, and sort of evil feel the movie and Donnie has. I have minor ADHD and no other know mental illnesses. I do however get homicidal/suicidal thoughts and urges randomly but none have ever been strong enough for me to act on. I have no idea why I do this and it may just be my random personality quirk but if its something more I'd REALLY like to know because it intrigues me. The whole idea is very interesting to me and if you have any idea why I might be doing it please share.