I have always enjoyed observing people, watching them in their natural or (unnatural)habitat. Sitting on a park bench watching children play, seeing an old man walk his dog, feeling a sense of envy as a young couple strolls by hand in hand. As a writer it is an essential tool to be able to observe people and to catalogue their actions, reactions, and interactions. To be able to blend reality with our imagination is what brings our fiction characters to life. Listening to teenage girls whisper and giggle, aching at the obese woman struggling through the mall, and keeping an eye on the latest fashions and hairstyles.
To watch and remember -- and take notes -- so when a particular character comes knocking at my writer's brain, I can breathe a touch of life into it's made-up soul. :)