Well, you want to have some questions of course. I mean, it'd be lame to just sit and stare at a guy, or just say "none that I can think of."
Although, "none that I can think of" may be a reasonable answer if it is your third interview and you exhausted your list on the first 2. Or, you can reask the questions and try to "get a different perspective" (i.e. a student vs. a dean, etc.)
Really though, be honest... Ask about crap you are actually interested in and that is actually important to you. I ask about culture, opportunities, student interaction and mentorship, etc. These things are important to me. I'd imagine that my interest shows... and hopefully that helps me out. If you are really interested in a TON of stuff. Ask questions until they stop you. They will politely tell you that they are out of time. No worries.