jtdMU2010 already wrote a ton of great tips so I won't rehash too much. But basically be yourself and be friendly. If the format is still the same as last year,
they'll do two 1-on-1 interviews for each candidate, and you'll likely get a student interview and a faculty interview. However, it is possible that you'll get two faculty interviews. Generally, my personal observation was that you get a "tough" interview and an "easy" interview, where you get grilled a bit more on one and the other is more conversational. The process may seem intimidating but in actuality everyone is very nice and helpful, their main goal is to get to know you as a person. (Since they've already got your academic stats)
During my interviews I got to talk a lot about my work with student government (I spent 2.5 years in those leadership positions), and my research. If you do any sort of research (especially if you have publications), make sure you know your topic like the back of your hand because they will ask you about the tiny details. They also want to get to know your personality and will ask you to describe yourself, etc. I was actually preparing for quite a few ethical questions but my interviews mostly asked about my research, clinical experiences and undergrad activities.
7 weeks in, I absolutely love Mizzou and the overall curriculum. We're two weeks away from our first test week (we have 8 weeks of class and then a week of testing, and a free week off) so I can't exactly gauge how much I've learned yet. Although there are frustrating moments, overall I'm a huge fan of the PBL curriculum, and it seems that everyone here really pulls their weight to help each other learn. One of my favourite aspects of Mizzou is the cooperative learning process; all the classes in year 1 are P/F, and there are no internal rankings, no comparative grading scales, etc, so people are truly vested in helping each other out.
I didn't go to Mizzou for undergrad, I went to a small liberal arts college in the middle of nowhere. Since we haven't had any sort of testing, I can't say how well my undergrad classes have prepared me for medical school. Generally, the atmosphere currently is that everybody feels a little bit overwhelmed at times, but for the most part people tend to get through it. The other thing is that I was a psych major, so for Block 1 (we're on a 4 quarter/block system) I feel rather overwhelmed because it is a biochemistry focused block, whereas I'm looking forward to Block 3, where neuro and social sciences are the main emphasis.
Sorry for the wall of text! If you have any more questions please ask!