Bring truthful answers to the questions:
Why do you want to attend the NA?
What do you bring to the NA?
Bring a thoughtful question or two. (One's that the answer to are not readily available in the literature.)
D's BGO interview was very relaxed and informal. We sat together for about an hour talking about the NA, watched a couple of videos, the BGO's career and family history at NA...more of a recruitment visit than interview. Then I left and BGO talked with D for another 1/2 hour while I sat in the car. She said he asked some questions about her HS career up to that point, classes she enjoyed, sports, etc.
Upon leaving, he told her to email him one thing she wanted the NA to know about her that was not in her application package or transcripts.
Relax - you'll do fine.