Hello,
I applied to both schools and at the end cancelled the interview at columbia due to overlapping with my final exams schedule.
I did attend to the interview at Harvard, which for the most part was very relax. The day breaks down into an initial talk about the school in general, the "New pathway" method, as they call it, of problem-based teaching. Then, you go in a tour around the school, lunch and finally the interviews.
You will have two interviews, one to one with faculty members, which for the most part are the same as that you'd probably faced in other schools. In general, they are ver amiable and polite.
In my opinion the school program is superb and the pass/fail system of grading certainly relieves pressure from the student and enables you to concentrate in what you like. First two years almost 100% with medical school, then you start learning the clinical aspects of dentistry.
I am sure you got the school brochure with the interview package, it is very informative and provides, for the most part, all the information you should know about the school before the interview.
I hope this is of any help.
Good Luck.
