Given the briefness of these opportunities I usually observe for as long as I can first, trying to draw in my mind before putting pen to paper. Usually once people notice they have become a subject I am done with the observation part, so I don't think it's too uncomfortable for anyone involved. It's much easier to look without being noticed, than to look-and-draw without being noticed, and I think it's good practice for the visual memory to divide drawing and observing in this way. In fact drawing in-your-mind only is something you can do anytime you want to make visual notes. Just look as if you really were drawing, following the contours and the forms. People get less mad this way, and it's amazing how much looking you can get away with!
