Hey...I had an interview at Columbia on Dec. 5. Like everyone else has been saying, it was very laid back.
Everyone meets in a board room for some "breakfast" and then we're given an hour or so presentation. Then the admissions committee comes in (all at once) and they call out the name of the person they'll be interviewing. Then you go off with your interviewer to a private room.
It was open file so be prepared to talk about your application. Also, my interviewer had a list of questions he was going to ask. Questions included: what's your proudest and least proudest academic moment?, why columbia? why dentistry? why not medicine? What can I tell the admissions committee on your behalf? Talk about your research experience? What do you do for fun?, etc.
I had a great interview and my interviewer and I hit it off really well. Unfortunately, I still haven't heard from them about an acceptance (although my interview was not even a week ago), so I don't know how important these interviews are...
Goodluck though. If you get in, please get in after me! haha jk.