my experience, 25 years ago, was that all the interviewers will ask you why you want to be a doctor. it sounds like a boring, standard question; however, there's more to it than simply "wanting to help others." they want to know how much thought and research you've put into this career decision.
did you speak with or shadow any medical professionals? did you volunteer at a medical facility? did you read medical biographies or other books about the medical field? did you have a profound experience as a patient, caregiver, etc?
a related question i was asked goes something like this: there are 3 applicants for every medical school position. if you are not selected for this medical school class what will you do? again, there are numerous ways to answer this question, but what they are trying to gauge is how much you've thought about your chances and your absolute desires. will you apply to nursing school, dental school, podiatry school, etc? will you work in a medical auxiliary field, such as phlebotomy, for a year then reapply to medical school next year? will you get another advanced degree?
or ... are you 100% convinced that you are such a strong candidate that there's no chance whatsoever of failing?
questions for you to ask: is the medical school focused on primary care such as family medicine? or to hospital medicine such as internal medicine, specialists, etc? are there any opportunities for research? rural rotations? (whatever it is you're interested in.)
i always thought it was a good idea to try to get a smile/laugh out of the interviewer one way or another. sometimes that would be to ask about an old book on his/her shelf. or inquire about his/her background. sometimes i'd inquire about politics. if you're daring, ask about their feelings on the affordable care act!
overall, don't be nervous. probably half the applicants will be sweating, reserved, brief, and generally forgottable. interviewers remember applicants who are smiling, friendly, gregarious. they need to see you as someone who is confident of his abilities, knowledgable about his career path, and pleasant to be around. just don't take things to the point of being arrogant.
i'll make one more small comment. make sure your suit & shoes fit comfortably. a lot of applicants have one outfit that they were on special occasions that doesn't quite fit right. when you're not comfortable it shows.