I just had my AROTC interview last week. My interview was actually very brief and relaxed. The PMS and I basically reviewed my resume and he asked me about specific activities I have participated in throughout high school. Be prepared to answer why you want to be a Navy officer, join ROTC, etc. I was also asked what my parents thought of my decision to apply for an ROTC scholarship.
Make sure you get a good night's sleep and clear your mind before the interview. Try to arrive at least fifteen minutes prior to the interview.
Like I said, my interview was for AROTC, but I'm sure your interview will likely be similar. Good luck!