1. I would have "done my homework" on the program's strengths and weaknesses before my interview.
Glad somebody finally said it
It's almost impossible to NOT have something to ask if you've done your homework. i.e....
--"I hear the program has X/does Y/offers Z. Can you tell me more about that?"
--"I saw that residents aren't given a conference/meeting stipend until PGY-4. Are there ANY opportunities for junior residents to attend a meeting?"
--"I read that residents take trauma call at more than one hospital. Are they all Level I centers, or is the experience mostly at one hospital?"
Try to get 3-4 questions ready in advance; you can always reuse them with multiple interviewers if you get stuck. Also, don't forget that your interviewers are rarely present at the "Introduction" talk given by the Chair/PD. If that's the case, you can always ask things that were answered there (i.e. if your prepared questions were already addressed).
Also, don't forget that attendings are people, too. You might want to ask what they think of living in Citytown, if it's good for raising a family (if that's an issue for you), why they chose the program, what they like about it, or what they would like to see changed.