While I was interviewing, I started off by thinking of questions in advance that I wanted to know, but I found that I often had time to get these answered in advance of the interview (at the many Q&A sessions provided throughout each day). Then, I felt somewhat awkward asking questions I already knew the answer to.
Instead, at the suggestion of my roommate (who is a PGY2 resident), I started letting questions come naturally during the interview and were much more personalized to the interviewer. This ended up being much more conversational and I felt like I was better able to connect with my interviewers (while still gaining valuable knowledge) than when I came in with pre-planned questions. Those interviews were also more fruitful for me.
I definitely would not stress about it, though. Just make sure YOU get some value out of the question and you do not sit there and say nothing.