It should go without saying that your demo reel should be your best work, but once you get the interview, how should you act?
I feel like being yourself is the best approach. It sounds like common sense, but sometimes you get nervous before an interview. Stay relaxed, be professional, and the rest will fall into place. Also, do some research about the studio where you are being interviewed. Having a general knowledge of the studio's work is a respectful way to approach the interview.