Have you ever noticed how some writers have a way with characters? You read their books and somehow even the random person they pass on the street feels like a real person. No matter how simple the scene is, the people who fill it all spring to life and flow in a way that just seems true to life. What’s the deal? How are they able to do that?
Obviously talent and skill have their place, but I tally this one as experience. Those authors took the time to people watch. Many great authors are doing this all the time. It’s where you can draw interesting quirks and inspiration. I recall an author who had written a scene that was fawned over by many. When she was asked how she had managed to make the simple act of waiting on a bus so vivid, she admitted the truth.
She wrote the actual events she had observed while waiting on a bench across the street. Faces were changed, names weren’t known to begin with, but the core of the living people was kept. Of course she cut out things, added others. It wasn’t exactly as it had happened since life tends not to follow a narrative flow. You do have to do some work after all!
Keep a notebook on you at all times. Sit in some public location and just watch how people move. Listen to how they talk to one another when they don’t think anyone is listening. Stay constantly aware of the nuances of human life. Write down what you observe.
Later when you wish to write something, you will have a wealth of source material to draw on. Go ahead and try it. I can vouch for its effectiveness.