Simple really. What are your interests? Research those interests before you get there, then at the interview try to get them to elaborate on what you've learned.
For example, I'm interested in International Health and Public Health. I looked at every school's website about international opportunities. Then during the interview (you could even ask both interviewers the same question, I did occasionally), I asked the interviewer what they knew about the international opportunities. As they started responding, I'd use what I had learned previously to guide the conversation into areas that weren't on the website. And often times, they don't know as much as you've researched. So in those cases, I would tell them what you know of. Tell them that you had hoped to learn more from the interviewer. Then ask them who or where you could find out more info. This tells the interviewer that you've done your homework, what you are interested in (like 10 years from now), and also let's them know that you are serious about the school since you will be contacting someone that they refer you to (yeah, right).
Works like a charm.... 🙂