If people watching were to be made a job, i might be the first to take it up. Creepy as it may sound [fear not, no stalker tendancies], quite simplistically said, it is very interesting. Imagine you're sitting with a friend or a loved one and there's silence and you ask the question, "what are you thinking". It's almost like that, except you don't actually ask the question out aloud and the answers are usually what you assume might be going through their head based on what they're doing.
It's not mind reading or reading your thoughts but rather just a simple attempt at deciphering based on expressions.If a picture can say a thousand words, imagine how much non verbal expressions and actions can convey.
1) I saw an old man in a wheelchair crossing the road and his hands were turning the wheels with such speed and it seemed like he was thinking to himself "I have the strength and ability to manage this on my own. I can do it." Ofcourse, in all probability i might be completely wrong and he could be swearing to himself in hokkien.
2) Watching a pregnant woman wearily plonk herself onto a seat and i imagine her thinking "dear god, i need to get this baby out now , Now, NOW and what the fuck are these assholes staring at, havent they seen a pregnant woman"
3) Kids are the best to watch. There's so much curiousity that you know they're constantly wondering about various things like how it would be to turn the big round wheel the man in the bus keeps turning, whether his/her mother would buy some chocolates, why they have to go to school, and so much more that i could keep going on.
It's a free form of artistic expression and it makes you appreciate the simpler things in life as you observe more, watch more and imagine more than the plain old boring self-contemplation in deciphering the meaning of your existence. Call me nuts, but i like it. It makes me a lot more aware about non-verbal cues given by friends and family which allows me to respond appropriately. Give it a shot. It's a good way to kill time if nothing else convinces you.