I've been to some interviews, both closed and open file. If the interview was a closed file, each interviewer explained why it is closed and the interview process in general. At the open file interviews, I was either told it was open, or I could see my application on the table in front of me. I haven't had a student interviewer, and I don't think that the norm is that a student interviewer is always closed file. I've been to schools where I got interviewed by 1 faculty, 2, and 4. I've heard another school is 5-6 depending on the day. Also, I had a two faculty interview which was closed file. It's different for EACH school. If they don't tell you before the interview or during, just ask before or during.
Also, a thick folder doesn't mean open; I had a closed with a thick folder of questions and sheets to write on and record scores such as excellent, good, etc.