For a few months I kept doing gesture and contour drawings of people and objects in public spaces. I went to a concert of Ron Carter, the great jazz bassist, and even while he played I sketched him, his guitarist colleague, and members of the public. I attended an informal play and sketched the heads, faces, and backs of theatergoers sitting near me. At a big party for my father-in-law's 70th birthday I drew guys dancing, guys laughing, guys asleep after drinking a few too many... Sitting at sidewalk cafes I drew passersby and fellow coffee drinkers. On occasion I rode the métro with a little notebook and discreetly sketched passengers.
Social convention would tell us that sketching people without their permission is a no-no, an invasion of privacy. So far I haven't had any problems with my public sketching. Either nobody noticed it; or if anyone noticed no one was bothered by it; or if anyone was bothered no one told me. It was another interesting lesson, to see that social settings afford a certain leeway of action; you can do more things in public than you normally assume. Another lesson, another liberation: shackles are self-imposed, including those of social behavior.
From the point of view of craft, sketching people in public is interesting because most of the time you need to finish the sketch within seconds of starting, before someone moves and the "pose" is lost. The brain, eye, hand, and heart all come together, and for ten seconds you're in the moment, intensely alert, concentrated. Little by little this attitude becomes your habit, and you remain always in the moment.
There's been an unexpected benefit to my sketching: It seems to me that as I become more observant, I also become more appreciative of people. When you pay attention and really look, people turn out to be quite fascinating. Everyone has a certain presence or energy; everyone has lived and suffered; everyone has something to offer to this world.