Open is okay unless you have some sketchy business in there. I'm very non-sketchy (see avatar) so they just brought up some random things to chat about...more out of interest than an interrogation. One had my PS in front of him with notes and highlights and guided a lot of the interview off stuff that was in there. Wait...my DAT scores are sketchy. That wasn't a big deal though. None of this "your GPA and DAT don't match" stuff people call a 'huge red flag' around here. Just read over your app before your interview so you remember what stuff you put (lied about) on there.
One benefit to not having an open file is that you have a little more control over the interview. It's blank slate and they know nothing about you. You can bring up stuff you want to highlight instead of them knowing everything about you already. You feel less repetitive also. If they have your file and ask you 'why dentistry?', you'll feel silly just reciting your PS when they've likely read it.
Done rambling. Need to study.