The interview start when you pull into the parking lot. You never know who you will meet as you go into the building. When my DS went to his MOC interview he was greeted by the MOC chief of staff. Although he is not on the board I am sure all the board's decision flow through him to the MOC. Just treat everyone with respect, as you would do normally.
Do not use their first name of the board members. Sir or Ma'am would be best as you stated.
You are not just interviewing for the academy, but to serve your country with a career in the military.
Remember it is possible that not everyone on your panel is Army, so don't disparage the other military branches.
Leave your phone in the car or give it to your parent to hold. Don't just turn it off, but get rid of any threat.
When standing keep your hands out of your pockets. In fact, take everything out of your front pockets, nothing worse than jiggling change.
Look after the details of what you are waring. This is your interview "uniform". Make it neat.
Make sure you have a question to ask the board. Most likely, at the end they will ask you if you have any questions. IMHO, no, is an incorrect answer. You can always ask "When will the [insert MOC] be making his nominations"
But most importantly be confident. You wouldn't be interviewing if you weren't qualified.
Best of luck.