I feel like program directors go out of their way to answer the most common questions during the morning overview. Does anyone know what are some good unique questions to ask a PD?
Unless you want to ask specific faculty about their research interests (not recommended, unless you know what you're talking about), probably the same sorts of things they are asking you:
What are the strengths/weaknesses of your program?
What are you doing to make the program better?
What made you want to join the faculty here?
Frankly, most of the interview should be focused on you. They want to see if you are someone they want. You, as an applicant, are assumed to want to be there by virtue of taking the interview.