Subway sketching is something that I love to do but you have to be quick.. Most of the time people like to have their picture drawn.. some don't. I think almost everyone likes to watch someone draw.. especially women.
The best are the sleepers or the gamers, usually I get about 1-2 minutes to get what I need sometimes I get 10-15 but thats pretty rare. I've had alot where I thought I would get a great drawing only to have the person leave or shift in their sleep, I guess it just wasnt meant to be...