In the past few years, human behavior and psychology become prevalent to developing my characters. I read about psychology, pay attention to the people around me, and try to understand why they do what they do.
Although motivations vary, real people in our lives, the ones that cause the most trouble, generally work from their weaknesses. These are the insecurities, fears, angers, and prejudices. We have them too, and sometimes they motivate our decisions.
This opens up so many choices of conflicts now open in your story plot. How will this character rise above their weakness? What realization or event will shake them out of it and provide them an evolution of their character? How will they grow?
A timid character must find courage in face of adversary. A boastful character learns humility through loss. A self-absorbed manager must overcome his own needs to help others, or lose his job. The potential of stories really opens up creativity.
Consider weaknesses. Ask yourself, what pet peeve do you have in behavior of others, and now write about a character who must overcome that flaw. You can also consider your own character, even minor ones, and write about how they will grow through your story.