What you wear depends somewhat on where you live. In some locales, suits may be "dressing up" -- in other places, your best jeans may be fine. Your BGO lives in your community and thus knows what appropriate attire is. If you don't own a suit, don't go out and buy one.
My personal view (and it's only that) is that the candidate should dress neatly -- nothing dirty or torn. Most wear pressed slacks and a collared shirt (or female equivalent). That said, the ONLY time I've ever taken a negative view of clothing is when I did an interview in a candidate's house and he came into the room in torn sweats, T-shirt and no shoes or socks. (I get that certain families/communities don't wear shoes in the house and that's fine -- altho this didn't appear to be one of those. But you can still wear socks so you aren't in your bare feet.)
He also researched who he was meeting with so he knew their background and work history . . . the interviewer was amazed that DS had thoroughly prepared for 'a casual interview' and stated that in 15 years he never had an interviewee conduct 'a background check' on him.
My first reaction -- EEK!
As for researching your BGO . . . first of all, you may or may not be able to do it. And, quite honestly, it could really backfire. Here's my opinion -- again, only that.
This interview isn't about me (the BGO). I'm not here to answer questions about myself. I'm more than happy to talk about my experiences at USNA, how that helped me in life, etc. But I'm very well might take offense if the candidate starts asking about things in my life that are unrelated to USNA.
In my intro emails, I provide my candidates a bit of my background -- that info is fair game in the interview. What would creep me out a bit is if I thought the candidate had spent hours googling me and started asking questions about or talking about parts of my life that aren't related to the issue at hand -- that candidate's application to USNA. Seriously.
As noted, the interview is about you. The BGO is essentially the face of USNA Admissions. Some candidates form close, long-lasting relationships with their BGOs. Others meet them for an hour for an interview and never see them again. Both approaches are perfectly fine. You absolutely do NOT need to be your BGO's best friend to have a great interview and to get a favorable writeup.
Don't overthink this, folks. Be yourself. Be honest. Try to make the interview a conversation about your interest in attending USNA because, in the end, that's what it's all about.