I would add that you should be prepared with some questions about the program/university/professor's research. My interview last year for the place I'm at included the standard questions about my (somewhat odd) background, what my plans were, why this university, etc, but ended with my questions. I was also asked how likely I was to attend if admitted, which caught me off guard, but it was a pre-decision interview. It was honestly pretty informal, but that probably depends on the prof.