If this is your first interview, good luck.
Actually, CCPM does 3 docs on however many students there are at once. So, of all interviews, this is the LEAST pressured. So it's a great one to start with. By your second interview at where ever else you go, you'll be much more relaxed. I kind of like to think of it as baptism by fire. There is only so much we can suggest to do or say or be or sound like. Just be yourself, DON'T LIE, and answer the few questions that there will be in a relaxed state.
When I interviewed there, there was 7 students plus myself. This was my 4th interview and I was definitely the calmest of all the prospective students. But I already had 3 other school interviews under my belt. I knew what to ask and what to say. A great way to learn what to ask is by listening to others and by listening to the students that will tour you around. Normally, they have the same students tour all the prospects, so they know what you are going to ask. When there is a lull in the conversation, they'll pipe up with something that they were asked recently or that they wished they would have asked when they were interviewing.
But as Eric said, the best advice is to pretend that you are going on a job interview. Dress very professionally, speak clearly, and try to encourage the interviewers interest in you as a person, not as Joe SixPack with MCATs and GPAs. You want to reinforce their decision to invite you for an interview. Make them remember you in a positive way. Besides, CCPM lets all their interviewees know if they have been accepted that same day. No sweat dude!