A big shout out to Bobby + Kei over at Imaginism studios...(check them out in the links) for introducing me to one of my now favorite pastimes. I started doing this a few years ago, and it's really the only life drawing I do nowadays. It's immeasurably educational...you learn so much about people, in addition to the fundamentals of proportion, form and character.
I have about 10 sketchbooks full of these...it's great, there aren't 2 that are alike. It's also my introductory course into caricaturing (a skill I have yet to master, though I'll get the hang of it someday). Most people never notice I'm drawing them, I think it's important to make sure I don't make anyone feel awkward...after they're the ones who are doing me a favor, I should be considerate to their comfort level. I learned some tricks though...choosing already occupied subjects (reading, talking, sleeping, etc), using periferal vision to spy on people until their attention is elsewhere, using multiple subjects for 1 pose (someone else's nose or mouth on the face). The important thing is to keep your momentum, be polite and be considerate.
Having fun with composition....