You will be asked why you want to go to that school. So, why DID you apply there? What kind of experiences are you looking for in a medical school? What is important to you?
Volunteer/student organizational opportunities... do you want to get involved with your med school class or do you tend to just want to do your work and go home?
Clinical exposure... When do you get it and what kind of exposure is it? What kind of patient population? Urban? Rural? Community or just in-patient?
Do you care about block classes or would you rather have semester long ones? Do you want a school with problem-based or case-based classes or just lecture? Are you interested in computer-based learning?
Facilities...computers, libraries, student lounge/area, labs, lecture halls... are they nice? Do you care?
Think about what you want out of a medical school and be able to address your issues. It helps to do a bit of research in advance about the school. So not only will you be able to tell the interview WHY you applied, but how the strengths of that school link into your own strengths and interests. You can also use the knowledge you already have about the school to ask specific questions of the interviewer. You can find out more about a school by checking out their web site or through books like Princeton Review's Best Medical Schools book.