I think all of the above advice is good. A few things I'd add:
1) For the love of god, do not worry about reading papers of each of the people you're meeting with (unless, of course, you're really interested in that person's work). It's completely unnecessary. I found reading the person's website throughly and the abstracts of their two most recent papers was enough to get an idea of what they were interested in, and the likely work they'd present to me.
2) There's a few questions you are guaranteed to be asked, so you should be prepared with good answers. Why MD/PhD? Why NOT just MD or just PhD? What do you see yourself doing in 20 years? What kind of research do you want to do? What do you do for fun outside of class? How do you feel about moving to xx place, and why do you want to go to that institution? And though I think they're technically not allowed to ask this, come prepared for the question "what other schools did you apply to?" It can be a bit jarring to be asked that one by the director of a program, but I think actually it often comes from a place of curiosity (the program directors are all friends/colleagues after all, I don't think any of them hold any ill will towards other programs for the most part in my extremely limited experience). For the last one, I was often vague and would say "Oh, you know, east coast schools" or something like that.
3) RE: that first question, every once in a while I (or friends) would interview with a PhD who would argue that MD/PhDs are pointless or generally refer negatively to the program. Thus, come with a good reason for doing the combined degree program outside of "medical research". And don't be put off by this attitude - if you stick to your guns and are consistent with your desire to do this program, you'll be fine with even the most negative of nancys.
4) Be flexible. There were times I went into interviews with people who didn't do research in an area I knew well at all, who asked me to contemplate hypotheses with them about their work. There were other times I went into interviews with people with research I was extremely interested in, and we'd wind up discussing queer theory or something totally unrelated to our science or medicine (which was relevant for my application materials, but replace with your favorite non-science topic). If you're flexible with what you talk about and don't come in with any pre-set notions of what you'll talk about, you'll be a-okay!
5) Honestly, be yourself. It's so obvious when people are faking a personality to get through interviews. These interviews are mostly fun conversations with interesting scientists, and if you've made it that far, you must be passionate and intelligent. Just let those things drive the conversation and you'll be a-okay!
Most importantly, relax and have fun. Remember that the interviews are just as much you getting to know the program as they are getting to know you. Take careful notes about what you like and don't like about each school (they all begin the meld together), and thank your interviews via email the day or two after!
