At the end of the novel Scout relays an articulation of one lesson she has learned through her maturation in the novel.
Scout tells her father that revealing Boo Radley's role in Bob Ewell's death would be "like shootin' a mockingbird."
This moment is a great example of how Harper Lee uses changes in her characters to convey important moral lessons.
Scout begins the novel without an ability to truly empathize. Though her time at school, especially with Miss Caroline, Scout is challenged (by Atticus) to learn to walk in someone else's shoes.
This notion of empathy is critical to the moral structure of the novel and is clearly articulated by Scout at the novel's end.
Another quotation that serves the same purpose comes from Atticus when he is describing Mrs. Dubose' battle to free herself from a morphine addiction.
As Jem reads, he and Scout witness the dying woman's battle against her morphine addiction and learn the true meaning of courage: "it's when you know you're licked before you begin but you see it through no matter what..."
Mrs. Dubose changes, inwardly and outwardly, through her experience. This change is emphasized by Atticus with its accompanying lesson in courage in the above quote. Her character is developed and deepened both in order to make this moral lesson clear and also as evidence of another lesson.
People are more complex (and often better) than they may seem at first glance.
This lesson is repeatedly expressed through character development, from Tom Robinson to Boo Radley to Atticus Finch.