You should think of both interviews as a job interview. First and foremost, be yourself. There are several threads on this forum, and I believe a sticky-- one comment that sticks in my mind is to treat the interview as an important event, both in attire and attitude. The interviews are not just a hoop to jump through.
The BGO is interested in understanding who the candidate is , and why he /she wants to go to USNA. I usually start the interview by telling the candidate that my goal is to paint a picture and fill in the blanks that the rest of the application does not address. (Keep in mind, we don't see the application package, so if there is something important you want to make sure Admissions knows -- Tell Us ! ) The BGO writes up a report and recommendation, and evaluates 6 factors - Interest and Motivation, Leadership Potential, Responsibility, Organizational Skills, Oral Communications and Physical Fitness.
Every BGO has their own style. It can be all question /answer or informal conversation. A lot of that depends on the candidates willingness to open up and talk to a complete stranger. I can assure you that the informal conversation is a lot more enjoyable than trying to pull information from a candidate that is afraid to talk. Be prepared to ask questions....one that always works is asking what he/she did in the Navy --most of us can't resist telling a sea story or two.