Create facts about the character's background and childhood. Describe the values he was taught as a child that shape how he thinks and behaves as an adult. Choose where he was born and raised, and whether or not he had a healthy relationship with his parents, siblings and classmates. Describe any traumatic events that changed his life.
Place traits within the character, especially the lead character, that the audience or readers can relate to, or at least understand. These traits will give the character a clear motivation for her actions throughout the story. A single mother with an obsessive concern over a daughter who has recently been diagnosed with childhood diabetes has a clear trait that will motivate her actions; a self-centered single mother's actions would be entirely different. A hero who is totally afraid of flying, but is forced to get in the cockpit of a runaway jet, has traits that may create an interesting story.
Give the character a distinct way of speaking and communicating. This separates and distinguishes different characters. Give the character a particular accent or an affinity for certain types of words, for example. Decide whether he is passionate and demonstrative with his hands while speaking, or calm and borderline shy. Make him eloquent and outgoing, barely understandable and shy, or somewhere in the middle.
Give the character a goal. Describe what he wants to accomplish or overcome in the story. This goal is the motivation for the actions that the character chooses to follow, based on his particular character traits.
Describe how the character looks and how old he is. Whether he cares for his appearance or perpetually looks disheveled says a lot. Whether he is devastatingly handsome or disfigured also affects how others look at him, and how he views himself. A 20-year-old man usually has a different outlook on life than a 50-year-old man.