You're almost guaranteed to have this in your grad school interview questions. So prepare a list of five or more questions. The best questions demonstrate that you've research your field and the school's faculty members in depth. For example, you can ask the interviewer to talk more about his or her research: "I read your article on _______, which is a topic that corresponds with my own interests, can you tell me more about it?" You can also discuss specific aspects of the school's department, facilities, courses, or other peculiarities that show that you're a serious applicant. So have your own list of grad school interview questions in your back pocket.