The BK at 14th and Broadway is pretty quiet. There had been a lot of loiterers at the front some months ago, but they were mostly older folk. This shop has an on-site security guard, as does the KFC at 61st and Telegraph.
When I've visited the downtown Oakland BK (not often) I haven't seen anything rowdy or strange, other than some kid who paid for his meal with money taken out of his (basketball?) shoe.
If you want to see interesting characters, check out the Carl's Jr. at 5th and Eddy in San Francisco (just off of Market Street, near Union Square).
I have been to both of those Carl Jrs. It’s pretty unsavory around there, for sure.