In real life, people are flawed. To create realistic characters they too need have flaws. These can be physical, emotional, verbal, sexual...
Create a scene introducing a character that shows their flaw or flaws. If you can't think of one consider the following:
*A young teen with crooked teeth
*A recently divorced person
*A child with a speech impediment
*A man who has learned he is impotent
For more on Well Rounded Characters check out my recent article at Inspired Author.