All great suggestions from Christcorp for helping to make that good "first impression" (he's obviously one pretty smart cookie, or at least he's been on a few interviews in his lifetime!
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One other thing to consider besides your appearance and demeamor, and it's been mentioned before: display confidence! And what is a good way to be comfortable in the interview? (well, at least as comfortable as you can be in that high intensity situation
) Practice! Get with a trusted adult (not your parents, the emotions would be too high ("Dad! That's NOT what they're going to ask! I hate you! You're always against everything I want to do", storms out of the room...
). Ask a guidance counselor, your CAP commander, a friend's parent you trust, or someone you know already in the military, and have them do a mock interview with you. Have them ask questions they would expect you to be asked in this type of interview ("So, why the AF Academy? What do you want to do in theAF? Tell me about what you expect to do?, etc") If they are unfamiliar with the process or the military, supply them with some starter questions and let them go from there. Be sure to include some "tough questions" that they think would make you think on your feet. Have them take notes on you, your answers, and the way you handled yourself. Only AFTER you're done, have them go over how you did (instead of interruppting after every answer to go over each question, which would ruin the pressure situation.
Am I telling you to go into your actual interview with practiced and rehearsed repsonses? No. The Board will see right through that if you're giving rote answers, and will hold that against you or try harder to trip you up. Instead, you should be prepared to confidently act on your toes. Some answers you should be ready for, but you should be able to handle yourself if that "So, how will yo feel knowing you may bomb civilians?" question pops up.
Most of all, RELAX (they ain't going to bite your head off), show some confidence, and KICK BUTT! Good luck!