I don't generally base a character off of just one person I observe, but I love people watching. I also evesdrop, though I don't tell anyone what I overhear. I actually learn more about human behavior that way than I do interacting with other humans a lot of the time. It's amazing what people act like when they think no one is watching.
Plus, people watching is a nice way to spend time... if you sit in the right place you can see every emotion in the space of an hour or so.