In these genres, the fundamental realities of a world can be anything imaginable: There can be wizards, or dragons, or intergalactic spaceships, or time travel, or dragon-wizards in time-traveling intergalactic spaceships. Nothing can be assumed. Which makes it mighty easy for authors to cheat by changing the rules whenever it’s convenient to the plot: “Oh, did I not mention that dragon-wizard time-travel spaceships are sentient and can crossbreed to produce baby spaceships? Well, they can.”
There’s nothing inherently wrong with changing the rules of engagement in the middle of a scene in order to provide an out for a hero in an impossible situation. In fact, here’s an interesting mental exercise when reading or watching the kind of stories where heroes get backed into corners: Note how rarely they think their way out solely with the resources at hand—the ones the audience already knows about—and how often they instead get away because something changes, whether it’s a new person arriving on the scene as a help or a distraction, an outside event that changes the shape of the problem, or just something the audience wasn’t in on, like a hidden weapon or ability.
And changing a story’s rules mid-stream can be an effective way to foster tension. Consider what happens in The Ring when Naomi Watts acts on what she assumes is the correct way to end the threat of Samara, and finds out too late that reality isn’t what she thought it was. Or what happens in Alien when the crew of the Nostromo sets out to capture an alien the size of a rat, and winds up unexpectedly facing something bigger than a human. Or consider the time-honored, annoying, but often-effective Twister cliché: When someone begins a story by saying “None of us has ever seen an F5 tornado! Never! That would be like the finger of God!” there’s a 100-percent chance that the characters are going to be facing that finger by the end of the movie. In all these cases, what makes the rule-change effective is the characters’ sheer terror at facing something outside of their understanding of how the world works. They think they know where they stand, and they act accordingly. Then they find out they’re wrong, and they have to figure out their actual standing in a hurry, with their lives at stake.
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