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It is quite common for a work of fiction to show a "Calm before the storm". Maintaining good humor after the tonal shift is much more difficult, and rare. The movie "Life Is Beautiful" is the firs...
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Source: https://writers.stackexchange.com/a/37226 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
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It is quite common for a work of fiction to show a "Calm before the storm". Maintaining good humor after the tonal shift is much more difficult, and rare. The movie "[Life Is Beautiful](https://en.wikipedia.org/wiki/Life_Is_Beautiful)" is the first example which comes to my mind. Much more common is that dramatic shift is serving for the purpose of characters' maturing up and losing their innocence (like in [Gone with the Wind](https://en.wikipedia.org/wiki/Gone_with_the_Wind_(novel))). So, don't worry about the tonal shift if your plot dictates it. Focus on your characters - do they stay realistic over the course of change? Someone can break, someone will get hardened, someone will laugh in the face of death.