The entire interview day is very laid-back, and the staff are incredibly friendly and informative. In the morning, you'll check in, listen to a presentation by Dean Saguil, , and tour the simulation center. During lunch, current USUHS students will be available to answer any questions you may have. Your interviews will take place in the afternoon, and you'll have two interview sessions with two different military physicians (some are USUHS alumni, others are not). The interviews themselves are very low-key, and from what I've experienced and heard from other people, you won't be asked super hardcore questions that are typical of "pressure" interviews.
However, you WILL be asked about your motivations and reasons towards joining the military. So be sure that you are able to thoughtfully articulate your reasons for attending USUHS and leading a subsequent career as a military physician. Other than that, just relax, be yourself, and put your best foot forward!
Good luck!