Once invited for an interview into the graduate program I was invited to spend 3 days visiting the campus, students, and professors. Over the course of these 3 days you will get to hear short presentations from professors and students regarding their research. This is a great opportunity to ask questions about their science and what their future directions and projects are. During the interview itself, be expected to discuss your previous research experience in depth. To keep the focus off of yourself, I would recommend asking the professor questions about their research, their field, and how they enjoy the campus and work environment.