I don't have a tonne of experience, but I would advise that you try to be conversational. By that, I do not mean use slang or anything like that, but do not sound robotic or rehearsed. I know it can be tough, but stay calm. Just pretend you are talking to a teacher or something like that... it is a formal relationship, but not one where you should feel intimidated.
Sometimes, try not to think it's an interview. If your friend's father asked you to tell him a little about yourself, you probably wouldn't have a hard time. So don't sweat it when the interviewer asks you, just answer the question politely (and of course, a little more job related than when talking to the father).
Try to inject a little humor (nothing crude), and demonstrate that you listen. For example, at an interview I went to, the interviewers briefed all of the potential candidates. During the briefing, they mentioned that they were tech people and not human resources people. Then before my personal interview started, they said they would be interviewing using the "behavioral" interviewing method. I jokingly commented asked if they were sure they didn't work for human resources, as that's pretty sophisticated for tech guys. We all had a good laugh, and it's good to "break the ice".