I had three interviews (for Brown, Columbia, and Yale), and basically the only advice I can give is be ready for anything. Each of my interviews was a totally completely different experience. My Brown interview was at a Starbucks, incredibly casual, the guy didn't really ask me any questions, we just had a nice 45 minute conversation, talked a little about Brown, about what I thought of my high school etc. My Columbia interview on the other hand, I had to go down to this guy's office and meet with him in his conference room. He asked rapid fire questions about my interest in Columbia, my academic interests, what activities I do in school, even some about unrelated things (politics, economy), didn't leave any time for chit chat, just asked question, waited to hear my answer, asked next question.
The only two important things I would say are ALWAYS have at least one question to ask at the end when they say, "do you have any questions?" I've heard this is literally the most important part of the interview - like, if you say no, they take a little note and it reflects badly on you, so even if you already know the answer, make sure you have a question. Also, make sure you have a really good answer for, "why specifically do you want to go to xxxxxxx" which will come up every time.
I wouldn't stress about the interview though. I've heard from a lot of people that it literally means nothing. Pretty much every interviewer writes glowing reports of the interviewee. I think it's really just to weed out the completely socially dysfunctional people.