Not sure how interviews work in the States, but I've been to med school interviews for Oxford and Newcastle in the UK. In case it's relevant to anyone, a few tips:
You want to make sure that you know the university and what the medical teaching program is comprised of. Oxford asked me about their policy on animal experimentation and whether I was comfortable with it, and Newcastle asked for my thoughts on it being a regional med school that may require students to make a lot of travels to various hospitals in the region for clinical placements.
You need to be able to show them who you are, your strengths and weaknesses, and what makes you fit to be (or want to be) a doctor. Make sure you know yourself well before they get to know you.
Everyone that gets an interview tends to have academic success, but not everyone is a deep thinker. You should be able to show them your thought process. Avoid giving short answers and put emphasis on how you reach a conclusion if you ever get a scientific question. For example, Oxford asked me why humans had two ears. I didn't know at the time. I thought it would have something to do with determining what direction the sound was coming from, and what property of sound the two ears could compare to do so.
Ethical issues are almost always asked about and people often give conflicting tips regarding these. I personally recommend going through all the points for and against each side. This will show the interviewers your rationale and how you justify things, which is what's important, rather than the actual answer. I personally didn't choose a side in an ethical dilemma and that might have made me seem reluctant, lacking the will to take initiative, possibly devoid of leadership skills.
Dress properly and have manners, but don't be a try-hard. Medicine requires you to adapt to various environments and personas, most of which can be quite challenging.
Don't smile TOO much.