Talking to some of my classmates from the class of 2012, it seems one on one interviews were given to applicants who made it pretty far in the USC admissions process in an earlier cycle but were ultimately rejected. This one on one is a chance to show them what you have made of yourself since then. It will probably be many of the standard dental school interview questions, along with things like "what has changed since last time you applied." Not sure how long they last.