Wear a suit! This is an important job interview. Show up clean shaven with a decent haircut. Think about and be prepared to talk about your goals and ambitions and how West Point and the Army are the best things for you (and the Nation.)
Read West Point Candidate Book by Sue Ross. There is a chapter on these interviews. Also great for nomination committee interview. The book is loaded with information on getting into WP, and other stuff like what to do after you get the appointment. She has the same book for USNA and USAFA too.
....My kid enjoyed those interviews, for each of the academies. They were more informational than anything. It was an opportunity to chat with someone who loved the school and was very familiar with the admissions process. The nomination committee interviews were different in that he was asked to answer questions designed to challenge him. He didn't feel that way with the FFR, BGO or ALSO interviews.
DS was advised the interview would take 2 hours, it took 3. He was advised to schedule the interview as soon as possible. He was able to schedule a Saturday interview for the very next day, 3 hours away. He arrived 15 minutes early and the retired Colonel was already there. He said it was informational, he wasn't asked very many questions. It seemed to be more of a recruiting visit than an interview.
I think these are as much a reality check as anything; everyone looks good on paper but some candidates are more self driven and this is a private forum to feel out how much the candidate really wants SA vs have they been pushed to apply by family or maybe just another notch on the 'top college' application list. Its about the intangibles that you cant see from a transcript.
We attended a USMA admission forum last summer and I recall the Major telling us how he was getting constant calls from a 1-star asking about his DS' application; never once heard from the kid. Motivation tends to come through in a direct interview without canned questions like they get in front of a MOC nomination board.
These interviews are a 1 on 1, 2 way exchange; good time for both sides to assess if its a good fit. Don't rehearse Q&A; just go in with open mind and be yourself. Ask as many questions as you are asked; its a golden opportunity to learn about SA and military life. My son was sold on USAFA after his ALO interview; officer was in guard / reserves and headed to Kuwait a week later [former B1 flight crew.]