I do these interviews all the time. Obviously, each interviewer is different, but here's what I look for:
1. Motivation. This is KEY. Not only should you be motivated to join the Navy, but you should also be motivated to be a Midshipman and future Officer. Make sure you know why you're in my office.
2. Good bearing/communication. Sit up straight in your chair, speak clearly and confidently, and be articulate and concise. Don't call me "dude" or "bro".
3. Be on time. Unless there's a good excuse, that will usually kill an interview for me.
4. Ask questions. It shows me that you're truly interested.
5. Do your research. Know what kind of careers you can select and what they are about. I don't expect applicants to know exactly what they want to do after they commission. If you at least know a little about SWO/Nuke/Aviation/SPECWAR, you will definitely impress your interviewer.
Hope this helps.