yeah, the md people are far more likely to be hard on you, but the MD/PhD interviewers can be somewhat tricky. in general, you get special treatment across the board and most of the interviews are very collegial and friendly. if you know what you want to research and know something about it, you'll be fine. usually the toughest questions you get are to explain your research past, its relevance, your research intentions, and again their relevance. the hardest situation i was put in by a MD/PhD interviewer was when he directly challenged the application of my research, citing financial impracticalities for big pharma, and asked me to defend it. otherwise, many of them are super people with whom you have some great conversations.