R-E-S-P-E-C-T. That's my reply when friends and colleagues ask how I approach photographing strangers on city streets.
There is a camp of "stealth" street photographers who shoot surreptitiously because they don't want the subject to pose or change their behavior. I think that's disrespectful and can be dangerous because some people REALLY don't want to have their picture taken.
It's true that initially subjects often smile and pose though only for a few seconds. Then, fortunately, they go back to whatever they were doing. Unless the subject is flying by on a bike or in a car, I always walk up and ask for permission or point to my camera and give the universal "thumbs up" sign. Usually I take 10-50 pictures and simply plan on deleting the first few.
This week's photo was taken at the annual "Dorchester Day Parade" in Boston. The participants were barely moving so I had plenty of time to select my "star" and position myself with the "supporting cast" spread out behind him. As I intended to stand three feet in front of him for at least a minute, asking permission seemed prudent! He gave the slightest nod of approval but never moved or smiled while I bobbed and weaved and snapped away -- hoping to show respect for his trust and consent.