The interview isn't too intense or anything (although I was super nervous last year). One person has seen your application and one hasn't. The interviewer who hasn't seen your application asks most of the questions and if you look at the interview feedback on here, you'll get a pretty good idea of what types of questions are asked. You'll find out when you get there if you're interviewing in the 1st or 2nd session, which determines when you tour. While waiting for your interview, current students sit at tables with you and talk to you about the school, answer questions you might have, and just try to make you more at ease.
The happy hour afterwards is pretty fun. If you go to it, some current students will take you over once the the last group of interviews are done. It's a great way to meet current students and get to know more about the school from them and just relax and ease your nerves left over from the day. A lot of people bring their pets, so there tends to be a bunch of dogs running around. Last year I think I recall current students even inviting us to a party with a bunch of their class later that night. For some reason, I think it was a Traffic Light party...could be wrong.