If I would give advice to anyone, I would say to be confident! Show them that you're human and not just a robot with good grades and a good MCAT. Be personable.
In my first interview we talked about fishing for literally 25 minutes. If you get the "Who are you?" question try to mention some of your hobbies, it could lead to a more conversational interview. My first interview was not an interview at all, it was just a conversation about fishing and I treated it as such lol.
EDIT: By the way, you totally should STILL fully prepare for the interview. Know your application inside and out, know your essays, read your personal statement again, know your research like the back of your hand, think of ways that the people that wrote your letters of rec have inspired you etc. My interviews were pretty relaxed, but you absolutely need to have all your ducks in a row before you walk through the front door for the occasional serious question.