I'm a bit of an observer... so yeah, I people watch. However, maybe it's my Fi, but I try to be careful not to let others feel like they are being watched as it does feel uncomfortable to feel watched. So I won't do any staring, and especially not with people close by. Interacting with friends, I think it's only natural to be observant of behavior, because actions DO speak louder than words. And you're a true friend if you can pick up on subtle clues of behavior.