So something that I did when I took my interview was just be myself in a professional and formal way. I answered whatever question they asked truthfully and thoughtfully, I basically expressed my true feelings about what they were asking in the question. The only thing I would do to prepare is to read your essay again and also talk with a friend or a mentor about the answers to the genetic questions (as stated previously in this thread). I was fortunate enough to have a practice interview with my fellow nominee and a member of our team who is a dean's list winner, but I feel that the best way to represent yourself is to be yourself. Your team nominated you for a reason, remember that and what was in the essay and you should be fine. I was told by a mentor that they run these interviews for the sole purpose of knowing who you are as a person, they already know most of what you did from your essay. I also have an interview coming up for the regional level and I am also pretty nervous.
Good luck, I know that you got this.
-MAR semifinalist From team 1923
(Sorry for any typos, writing this on an iPhone)