I've only been to one at Mt Sinai. But the format is standard for faculty interviews - usually one to one, sometimes a few students to one prof. You meet with the profs you are interested in back to back and talk to them. Each prof has a different style, some like to talk about their research, some will ask you about your research, why you want come to the school, where else you applied, your career goals. There are also social events to interact with graduate students, alumni etc. It's as much about you getting to know the school as them getting to know you. During the whole process, you are kinda being observed. Well, not in a big brother 1984 sorta way, but just don't do something stupid like get completely drunk, say something inappropriate etc.