"People-watching" is something that most people like to do. When you're bored, waiting in line, sitting in the mall, or in my case, waiting at the bus stop for a bus that never comes, it's easy to start paying attention to the trivial mannerisms, conversations, and randomness of humanity. To put it in perspective, at some point in your life, someone has done this to you.
For an artist, humanity is like a buffet. Character and story ideas are absolutely littering the streets. A good artist makes a habit of people-watching because they know that there is much to be learned from the way a person behaves naturally--when they don't think someone is watching. This isn't just good for story and character ideas, but also simple and subtle things people do when they walk, sit, talk, etc.
It was a little chilly when I was sketching these passers-by, and now that I look at this sketch, I can only hope that you can see the details I was trying to capture. The two that I learned the most from were the first and third (beginning on the left) figures. The first girl I saw pass by was crossing her arms in an attempt to stay warm; I wonder if that's connected to the fact that she was wearing a skirt! The third person was actually in a group, and as he passed by, I realized that I've seen tons of guys walk just like this: chest out, chin up, hands in the pockets, back straight. It's kind of a "I-spend-my-weekend-nights-at-the-club" way of walking, if you ask me.
When creating characters, capturing details like these are massively important for creating believability. Think of the people you know best. You can instantly think of how they walk, sit, explain, listen, etc. What's more important is this: think of the people you love and care about. Isn't it the little details about their personality that many times you love and hate the most? It's those seemingly unimportant details that define characters.