Having gone through the process of applying to MSTPs last year, I can definitely relate to your feelings of anxiety and loss of focus. I actually ended up missing 3 weeks straight (and probably 4-5 weeks total) of undergraduate classes in order to interview at a bunch of schools during one trip. First off, it's still early in the process, so don't get worried about not having too many interviews yet. Instead, focus on the preparation.
Main things you need to be able to answer:
1) Why MD/PhD and not MD or PhD alone?
2) What are your career goals?
3) What do you do for fun?
4) Tell me about your research...
You will need to be comfortable discussing your project(s) and your role on them. They are looking for evidence of initiative, creative thinking, motivation, risk, and passion for science. These things cannot just be stated, but they need to come across as very obvious during your interviews by HOW you talk about your experiences and goals. If the interviewer detects any doubt about whether you want to do the joint program, you'll likely be in hot water. They are very attuned to screen out for people looking for a "free MD." The interviewer also wants to see that you balance your hobbies and interests well with academics, as multitasking is a very important skill in those attempting careers in both medicine and science.
My advice would be to do some mock-interviews with your research advisor(s), medical professionals, students, or friends. The more practice, the better. Believe it or not, there will come a time when you will be sick of repeating the same story over and over again. You'll wish you had a tape that you could just re-play for your various interviewers.
Also, be aware that physicians and scientists come from very different perspectives. Some interviews will be for the MD program, while others will be your MD/PhD program interviews. You might have to adapt yourself to the particular situation in order to effectively connect with the interviewer. Just think of this as practice for the rest of your life as a bridge between medicine and science.
One thing I wish had been more aware of going into the process, is that you'll often be able to meet with faculty you're interested in during informal interviews, which will have to be scheduled. Take advantage of these opportunities to learn, ask questions, and demonstrate your curiosity.
Also, the dress code is basically the same as regular MD interviews: suit and tie. If you are interviewing over the course of a couple days, then it is probably best to bring extra shirts and a couple of ties so that you don't look like you're wearing the exact same thing.
In all, MD/PhD interviews tend to be a little less formal and more of a discussion of research than anything. Your interviewer often will ask if you have any questions for them, so be prepared on this front as well.
Well, these are just some tidbits of info that I hope will help you during interviews. If you have any other questions, please feel free to post and I'll try to answer as soon as I can (between classes, labs, etc).
