He/She should (be) -   

1. well dressed.
2. natural and normal
3. tension free
4. confident and cheerful
5. show interest in the interview
6. give positive and constructive response
7. optimistic
8. (response) clear and to the point
9. convince the interviewing officer through his reply
10. prove through his answers that he is suitable for an officer in armed forces.
11. In case he does not know the answer, it is better to cut sorry figure instead of managing the answer, where he is not confident to reply as correct
12. Hear the questions carefully and giver answers to the point and relevant to the subject
13. Not pose himself superfluously and should not hide the facts.