A subtle and efficient way to give a character "life" is through the setting that you place him in. You want to place characters in scenes that provide insight into the character's personality or predicament. For example, in "Twelve Years A Slave" by Solomon Northup, readers learn more about the main character Solomon when they see him held prisoner in William's Slave Pen. It's in this scene that Solomon is revealed as an optimistic person who has the ability to quickly observe the nature of others. This scene also foretells of the hardship in his future.
A character should have an idea of herself, and this is conveyed by the words she uses and through her actions. For instance, if a character believes herself to be an upcoming movie star, then her dialect should reveal this and her actions should confirm it. Another way to term this type of character development is defining character status. A method for establishing character status is to allow your character to hold conversations where her status is challenged, even if done in subtlety, Steven James says in his "Writer's Digest" article, Raise Your Characters Above The Status Quo.
As in life, not everyone you meet will leave an emotional impression on you. There are, however, people who do. When developing your characters, factor in the type of emotional reaction you want the reader to have. While a reader may initially forget a character's name, he won't forget how a character made him feel. Emotions are readable by facial expressions, body language and words. Use these methods to give your character emotional depth. Be careful with this character developing technique, however, so you don't overwhelm your story with characters who all cause strong emotional responses, Northern Illinois University's Writing Across the Curriculum program advises
Very few things are more telling of a character's personality than the decisions that she makes. Give your character a life of her own by giving her circumstances when she has to make a decision. Allow your reader a window into her psyche by observing how your character chooses between driving or catching the bus, eating an apple or a bag of potato chips or cursing or using polite language. All of these instances, Texas A&M University's Writing Center states, build your character and simultaneously offer the reader a chance to form a judgment about the type of person she is.