I just had mine last week. The interview portion is extremely laid back. You start the day with a question and answer session with some of the first year students. We were then split into two groups, one which interviews first and one which tours first. I got to tour the school first and get many of my questions answered by the first year student who was my group's guide (the groups all had either 2 or 3 students). I then went for my interview. It was an open file interview, so the professor knew my background. It was fairly informal. I actually don't remember most of the questions asked since it seemed more like a conversation. I do remember being asked which class I had found to be the most difficult. As with most interviews, I don't believe that they are looking for the answer itself (its not like if I had said Calculus is the most difficult I'd get in but if I said Biology was then I wouldn't), its about how well you can carry a conversation and express an idea. Since optometrists need to be able to communicate well with their patients, they are interviewing for that aspect as well as for personality and motivation. They want someone who really wants to be an optometrist and has an understanding of the field.
But don't stress about the interview part. I didn't think that it was anything to get nervous about. My recommendation is that you look at it as your interview for them. Obviously, you don't want to be cocky, but you are finding out if they are the right fit for you as much as they are assessing your fit with them.
All in all, they said that the day would feel very long but it was only about 4 hours. Breakfast and discussion with students, then tours and interviews, then lunch in smaller groups with students, then a tour of the clinic (which took longer to get to then the tour lasted).