I felt the most important question asked in MD-PhD interviews was "describe your research". What's most important to realize is that the majority of your interviewers that serve on admission committees will not be experts on your exact area of science; you need to be able to describe your research to those who, from your perspective, are "lay people". You need to try your hardest to come off as passionate to the point where your research actually sounds like fun (in my opinion, a good test is to see if you're capable of getting an elementary school student interested)! We all realize that not everything about research is always enjoyable, but no one likes to hear someone drone on and on and put on a pity show about how your experiments never seemed to work or how your senior lab mates or PI would never have time to talk to you...Rather, accentuate how you were able to overcome all the roadblocks you ran into and how exciting it is to constantly be challenged to think by nature!
A question that came up pretty often for me was the ever-dreaded "tell me about yourself": you have ~20 minutes to say whatever you want to the interviewer. Have something ready for this! I did and it saved me on my very first interview! You probably also want to talk about your research in a way that highlights your passion and ability to reason through problems scientifically.
Something else I saw commonly was to discuss how you envision having an MD would actually help you with research, and how a PhD would help you with clinical medicine, if at all. Be ready to talk about what you think each degree means and what the relationship between them are.
You should also be ready to describe where you see yourself in ~10 years. One interviewer put it like this: if you were given a crystal ball and could get your deity of choice to make what you see in it a reality, what would you want to see in the crystal ball?
Although MD-PhD interviews focus mostly on research, it's also important to be prepared for "standard" MD-only interview questions. Make sure you know why you want to do medicine! It's harder to articulate than one might expect. I think it's also a good idea to familiarize yourself with medical specialties at a basic level. Even though residency is so far off for MD-PhD applicants, a pretty natural question to ask someone who wants "MD" after their name is "what kind of doctor do you want to be"? It's okay if you have no idea (and honestly no one does as a pre-med; just be able to articulate this well), but you should know what the difference between an internist and a surgeon and a pathologist is. I personally felt quite silly when I was being interviewed by a dermatopathologist and had no idea what she did for a living. I didn't admit to my ignorance out loud (it probably showed, though) and had to "wing" parts of my conversation. It worked out anyway with an acceptance at that particular school, but I wouldn't recommend doing what I did!
Lastly, I found almost all of my MD-PhD interviews to be fun and low-stress. Even WashU's crazy committee interview was enjoyable! Your interviewers really want to get to know you better (they invited you for a reason) and for the most part are very nice individuals. Note I said "for the most part": be prepared to run into the occasional "nasty".
Feel free to ask if you want to hear any more! Good luck!
