My Pre-Med advisor had a great book in her office. I think it was called something like "Essays that will get you into medical school." The book as 2 chapters on interviews. It tells you how to prepare and what to expect. Also, does your university have some kind of career center? Some of them will do mock interviews. If not, ask a friend who is mature and familiar with the med application process and have him/her do a mock interview. From my personal experience, my interviews were very conversational. They all started off by asking me specific questions from my essays or from my application. They then just kept asking questions that seem to naturally come up from my previous answer. There were very few "planned questions." If you get that book, you'll see what I mean by "planned questions." Some examples are:
1. What are your strengths/weaknesses?
2. Why med school?/Why medicine?
3. Do you know what you're getting into?
4. What are you passionate about?
5. What are your hobbies besides academics?
6. Tell me about yourself.
Aslo, be up on current events. Know about the new medicare bill and the obesity epidemic, for example. No one has ever asked me anything really specific so I think they just want to know that you are aware and have thought about these issues.
Best of Luck and I hope it goes well! Most of all, don't be nervous cause I think most doctors are really nice. They just want to get to know you. It's not supposed to be a cut-throat experience.