Sitting slightly behind someone is another way of getting to draw them without being beaten up, er, noticed. Along those lines, I was noticed drawing someone on the train today, but not by the person I was sketching. The subject was a couple seats forward, asleep with his head back. But when people got off the train, someone walked past me and said, "Hey, is that the guy up there sleeping?" When I said yes, she said she'd have to watch out if she saw me on the train. Or you could pose, was my answer.