Usually they'll discuss your research with you. If you know it well, you should do well. This discussion can range from sort of a general overview of the subject and perhaps probing your broader knowledge of it, to going into the technical details of your experiments. Either way, if you've worked on a project for a number of months, you should be able to do fine.
Often, the interviewer will ask you why you are interested in a field, e.g. immunology, neuroscience. Be sure you have a good answer as to how it connects to clinical medicine.
Many will also ask why MD/PhD? Just explain that intelligently.
Many will also ask why you want that particular university. My response to that was discussing their research in general, or a particular department as being quite strong, or the possibilities of collaborations, etc.
Sometimes they may finish quite early, and ask if you have any questions. Unless the interviewer is someone intimately involved in the administration of the MD/PhD program, or has had MD/PhD students, it is best not to ask logistical questions, such as years, stipend, and the like. It is much better to ask them about their research (people love talking about themselves), and ideally find something interesting about it and perhaps a connection to your own field of interest. Keep up your attention during this portion of the interview, and feel free to interject and ask good questions. That's a plus.
Occasionally, you get someone who wishes to discuss their research almost entirely. I've had two interviews like that so far (of several dozen in all). In such cases, just ask lots of questions and show interest. Such people are more into selling themselves than testing you.
Rarely will you get someone who grills you on a completely unrelated topic. My research was in immuno and something neuro-science related, and one of my interviewers asked me about my college education and then went straight to systems biology, why networks are problematic, how I would fix it, what thesis topics I would envision, etc. That was kind of a rare interview.