The Darth Vader reveal works because Star Wars limits how many Jedi exist and keeps Luke ignorant of his father. Nicolas Cole explains how the best twists feel shocking and inevitable at once, fitting the story's own rules rather than breaking them.
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One of the greatest surprising but still believable choices ever was in Star Wars when Darth Vader admits to Luke that he is his father. This is a great example of a choice that is both so surprising and yet still believable at the same time. It's surprising because the story has clearly pinned Luke and Vader against each other. You wouldn't think that these enemies were actually related— they're family. But it's also believable because of some of the limitations of the story. There are only so many Jedi in the world. In hindsight, some of the makes sense. Lute hasn't known the identity of his parents, he just knows that his father fought in the wars and was a powerful figure. So there are these little Easter eggs dropped throughout where by the time this choice lands, it's not so far outside of the realm of believability. This is the line that you're straddling whenever you're telling any sort of story. It doesn't even matter if it's nonfiction or fiction. Memoir is a great example of this. The details that you choose to share need to sit within these constraints. It has to be surprising and unexpected and yet also fit with within the world of believability and be the thing you expected the character to do, which is a really funny duality to balance.