If they take the time to interview you then you have usually gotten past the MCAT/GPA hurdle. There are a few exceptions such as where I am at FSU where they are really looking for people to fit the mission, so they might interview you and then see if you raised the MCAT to a certain point.
If you are waitlisted it usually means you weren't that bad but you weren't that great at the interview either. I wouldn't start sending letters of commitment yet, but I'd keep them updated and let them know you are very interested in the program. Bust your ass and do well in classes for the semester, continue volunteering, work hard, and do whatever else it takes. Don't just sit there and say "Welp, I'm on the waitlist" and sit around twiddling your thumbs. Try to be proactive.