I know it sounds cliche, but just be yourself and don't sweat the interview. I felt both of my interviews were very high pressure (and I interviewed at ~ 8 other schools), but I just rolled with the punches. Our class is really diverse and there is a wide range of MCAT's and GPA's. That being said, most matriculates seem to fall into the extremely high GPA + median MCAT category. That is probably a bit of school preference and a bit of candidate preference. My best advice to prepare for interviews is to read the previous interview feedback here on SDN, as I found it very helpful.
As for stats, last I checked it was around 4-5k applications, 1k interviews, and 230 matriculants. I would guess they accept somewhere in the neighborhood of ~400 so, if you get an interview and do well you have a decent chance of matching. Overall, UTMB is an amazing school and probably the most laid back student body in the state. Thankfully, I brought a couple of gunner friends with me so that I would have someone to study STEP with before orientation even started