This site currently has a friends tag and a close-friends tag. Yes, close friends is slightly different from friends. But tags exist to help people find questions. Are there really a lot of people who want to answer questions about close friends but not friends? Personally, if I was interested in answering questions about friendships, I wouldn't care that much about differentiating between the two.
A perfect example:
To close friends
Move closer, and hint that their breath smells without drawing others' attention to it.
To not-so-close friends
Probably stand a little further away, and hope they sense why I did that.
Depends on how friendly we are. Refer the abovementioned.
No limit on what I'd say; but not in public though. No formalities.
Don't mention it. I'd do what I came to do, and move away.