Some observations about character arcs, after reading probably too much fiction:
A flat arc can be as powerful as a change arc. Atticus Finch doesn't change. The world around him does. His steadiness is what makes To Kill a Mockingbird work. The town reveals itself, and he just stands there, being who he already was. That's the whole point.
Characters who change too fast didn't believe what they believed in chapter one.
I keep coming back to the idea that the arc which actually matters is the internal one. External events are just pressure. A storm doesn't change a house. It reveals where the foundation was weak.
Elizabeth Bennet in Pride and Prejudice is wrong about almost everything in the first half. Her arc is the slow dismantling of a very smart person's certainty. Austen makes you admire Elizabeth's intelligence first, then shows you how that same intelligence built walls around bad assumptions. It's a brutal trick if you think about it long enough.
What a character refuses to acknowledge is often more revealing than what they confess.
I'm genuinely uncertain whether the most beloved characters in fiction actually change much. Sherlock Holmes doesn't. Atticus doesn't. Poirot doesn't. Yet readers follow them through thousands of pages. Maybe we say we want character growth, but what we actually want is to spend time with someone whose way of seeing the world is more interesting than ours.
The character who changes too cleanly, the villain who repents fully, the coward who becomes fully brave: that's a wish dressed up as an arc. Real people carry their contradictions forward even after they've "changed." They backslide at Thanksgiving. They revert under stress. The old self doesn't disappear. It just gets quieter for a while.
Tolkien's Frodo keeps coming back to me. The ring doesn't corrupt him the way it corrupts others, but it costs him anyway. He saves the Shire and can't live in it anymore. The arc is about what a good person loses when they do the necessary thing. Most writers would have let him come home and be happy. Tolkien knew better.