Observing the masters
Actors: by all means watch the actors you like and learn from them, but if you really want to learn how to play a human, watch a human--not a human playing a human.
Get on the bus or train. Go the mall. Watch people living their lives. See how they argue about paying for something at a store. See how they try not to look like they've been fighting in the car. See how their eyes light up when they see someone that matters to them. See how they try to disguise a limp. See how they respond to rudeness, kindness, indifference, indigestion.
Watch people being people. It's authentic. It takes off the veneer of artifice and replaces it with real reactions to true stimuli. It's an invaluable tool for the actor.