Become a people watcher
One of the best ways to depict realistic, well-rounded characters in your stories is to become a people watcher. Are you one? If not, try it out.
Go to your local mall or someplace else where people congregate, find a comfortable bench or chair or seat on the grass and just watch people. Watch how they walk, the rhythm of their gate, the swing of their arms, the posture of their shoulders, the wag of their hips. Watch how they interact with each other, facial expressions, how they laugh (open-mouthed, or tight-lipped), if their eyes smile with their mouth, if their expressive in how they talk or reserved, if they use their hands a lot. Watch thier body language, how they stand, what they do with their hands (in their pockets, behind their back, wringing them like a dish rag), what they do with their legs (crossed, open stance, narrow stance).
Watch to see if you can pick up on their mood by what you observe. Watch to see if you can pick up any quirks in their mannerisms: Is she always tucking that loose strand of hair behind her ear? Does he rub his chin excessively? Does she click her pen repeatedly? Does he cover his mouth when he laughs?).
And if you really want to make it a learning experience, take along a notepad and jot down some notes about what you observe. Describe the person you’re watching. Describe how they move, talk, walk, stand, sit, run, whatever.
The next time you’re in public, don’t just go about your business in a tunnel. Take some time to sit and watch people. They really are fascinating and you can learn so much from them that you can then incorporate into your writing.