Okay, I never applied to Columbia, but I think one of the best ways to prepare for ALL interviews (at least when you are starting out) is to do a couple of mock interviews. You have a week and a half until your interview, so yes, you do have time. Ideally, try to find a med student -- should be easy if you are in an area with a med school nearby. Otherwise, ask a friend, professor, or family member who you trust. Show them the interview feedback stuff, give them a list of sample questions, etc.
If you really are convinced that you can't do a mock interview before you go to Columbia, read over your application thoroughly -- EVERYTHING on there (primary and secondary application) is fair game to be talked about during the interview. THINK about how you would answer such broad questions as "Why medicine?" "Where do you see yourself in 10-15 years?", etc. If you have any red flags on your application, THINK about how you will discuss that if asked. I'm emphasizing the word "think" because you don't want to write out your answers or anything like that -- you don't want to sound staged or rehearsed. Finally, try to come up with at least 5 questions to ask during your interview about the school, and try to make them somewhat unique to Columbia. When at a loss, my favorite stand-bys were always to ask the interview what they liked most about that school, and what they would most like to change or improve about the school. Tend to get more interesting answers/discussions than the standard "What options do you have for research," etc.
Relax, sleep well the night before, and good luck!
