Character arcs

A story arc is the inner journey the character takes throughout the book. They start one way, and due to things they have experienced and/or learned, they are different by the end. Perhaps a young woman starts out being an immature, silly person and by the end, she has gained wisdom and maturity. HOW this occurs is why we read the story, why the story exists in the first place. The more at stake, the bigger the change, the bigger the character arc. Editors and publishers like big character arcs, but as with anything, it is harder to write a powerful character arc than you might think. The reason is that the more a person needs to change, the more annoying or immature or whatever they are in the beginning. But if the character is too unlikeable in the beginning, the reader may not care enough to stick around to see them change. Added to that, we need to maintain a "trueness" about the character and a consistency to keep them realistic.
Remember Scarlett O'Hara? She was a spoiled brat in the beginning, but we stuck with her because every once in a while, the author would reveal something vulnerable, some sort of weakness about Scarlett that made us identify with her. And she was strong...something we all wish we could be. Much about her personality didn't change very much throughout the story (she was consistently selfish), but here and there we see glimpses of her growth. Like when her daughter is killed or when Melanie dies, we witness her pain although she hides it from the world. And in the end, the author (Margaret Mitchell) left us with some hope that Scarlett did change, that her character arc had come to an end with Scarlett's last words, "I'll think about it tomorrow.". What was genius about this story was that Ms. Mitchell let the reader determine Scarlett's character arc for themselves.