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I think there's a difference between character development and character depth. Development means change. You can have an interesting villain who is only ever a villain, but still has backstory, mo...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/17241 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/17241 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I think there's a difference between character _development_ and character _depth_. Development means change. You can have an interesting villain who is only ever a villain, but still has backstory, motivation, relationships, and hobbies. That's a deep character who doesn't change. But if your character acts like a boring, shallow buffoon for two acts and then suddenly gets hit with a Plot Ball and turns into [La Maupin](http://www.rejectedprincesses.com/princesses/julie-daubigny), she's not going to be believable, and your readers will wonder where this fascinating spitfire was for the first two-thirds of the book. Unless there's some compelling, in-character reason that _she_ deliberately wants to hide her candle under a bushel, try to dole out hints of who she really is from reasonably early on. You don't have to give away the whole game, or reveal her to the other characters, but the readers should realize there's more to her than what the other characters think.