“The best way to understand people is to listen to them.” Ralph G. Nichols
Closing your mouth, opening your ears, and opening your mind is a great way to gather a ton of information. Observe with ears and eyes and try to feel what they are conveying on an emotional and intellectual level. Try to see not just what they are clearly showing but what is not said, the subtext to the conversation that really requires focus and rapt attention.
Really listen. Actively listen. Use all of your senses and capabilities to “get” them.
You’d be amazed at what you can get out of it.