You might also want to keep in mind that if you are sent off, to dinner or whatever, to spend time with their current grad students to ask them questions, etc., that you still need to NOT be an asshat. I went to one of these once where the interviewees went to eat with the current students, no professors, and one applicant brought up that he didn't think the US should help Haiti after the earthquake, he said he 'hated Mormons', etc. Just obnoxious stuff. (One of the grad students present was an advanced student, very respected by the faculty, and a Mormon.) Just be aware that the students are there to answer your questions, but they may also be asked their opinion about you by the faculty after you leave. I don't know that this is standard and how much weight it may have with your application, but if you say any random thing to a student keep in mind that they may repeat it to the faculty. Sorry if this seems blatantly obvious, but clearly some people don't consider it. Time spent with their grad students should be considered part of the interview so to speak.