In Downtown Crossing just now, there were 5 bicycle cops gathered together on the sidewalk beside a police car with its lights flashing. A small crowd was gathering around the police car where there was someone handcuffed in the back. A lady, presumably the arrested person's friend, turned to the 8-10 person crowd and said, "There's someone in there, and it ain't nobody's business....unless they call you!"
I have to say, she hit the nail on the head. Probably comes with experience.