My DD took her BGO interview very seriously. She scheduled a mock interview with the father of a classmate, and she found it hugely helpful to have practice fielding random questions (some of which actually surfaced at both her BGO and MOC interviews). She wore dress slacks and a tailored blouse, and her hair was in a bun - not a uniform, but representative of how she will dress as a naval officer. She practiced smiling, shaking hands, making good eye contact, not raising her voice at the end of a statement, and formulating answers in her head rather than saying, "Um..." She also developed a list of questions that showed she had done her research about the Navy and the application process. In the end, her BGO said, "I hope you have your MOC on Saturday, because I'm on the panel that day!" And sure enough, she did, and she knew her BGO had her back. And sure enough, she received her nomination.
So, of course, try to be relaxed and confident, but don't confuse that with casual and cocky...this is more than a hoop to jump through - it's an opportunity to gain a partner who may just make the difference as to whether or not you receive an appointment.