Most importantly is why you want to attend a service academy. I would not respond with "because it's free," or because "I'll get a good education." Reason being is that you can get a good education at a civilian college. I would discuss your desire to serve as an officer and desire to lead others. Another typical question is to discuss your strengths and weaknesses. Do not tell them that you do not have any weaknesses. That is a lie. Also do not tell them something like "oh I have a hard time getting up early" or anything extreme like that. Try to turn your weakness into something positive. For example "My weakness is that I am impatient because I am a very results driven person and like to see the outcome of my hard work as soon as possible" or something else that you can actually spin into something positive. Do NOT lie. Trust me, they'll be able to tell if your trying to b.s them. As far as the reason for wanting to attend the service academy, you should think of exactly why you wanted to apply in the first place. A good starting point is to look at your response to the candidate statement that asks the same question.
There's a thin line between sounding sounding cocky and sounding confident in the interview. Speak of your accomplishments confidently but don't flaunt them either.
