Open file means your interviewer has access to all the material in your AMCAS, secondary, and recs. It doesn't necessarily mean they'll have read it all before interviewing you, but if you have a particularly conciencious interviewer it is possible. Closed file means they have your name and nothing else. It's supposed to make the interviewer talk to you rather than trying to see you as a function of your numbers. I know at Rochester, the interview is closed file, but when you get offered an interview they make you write two half page essays on different topics which the interviewer gets, so there is some leeway on what a closed interview could be.