It is a closed-file interview, so pick something they might ask you to explain if they had had the chance to look at your file beforehand, and then explain it. I don't know what one would say if one had a perfect record, but I certainly had plenty to talk about. I have a C on my transcript, in biology no less, so I explained that I had gone to an arts high school, hadn't really had any science before the class I got a C in, and then pointed out that I ended up majoring in biochemistry with an OK gpa. I think that's about what they're looking for...?
Good luck!