The Rule
A compelling narrative requires an increase in intensity
While it's true that few books go from 0 to 60 with no ebb and flow, each major point of a compelling narrative heightens the tension, the stakes, the difficulty from the previous one. A gangster who robs a gas station must next try to rob a bank, a hero who fights a goblin first must next fight an ogre, and so on.
See Kurt Vonnegut on "The Shape of Stories".
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