Chekhov's Gun and Slow Horses
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Chekhov's gun
Chekhov's gun is a narrative principle that states that every element in a story must be necessary and irrelevant elements should be removed. For example, if a writer features a gun in a story, there must be a reason for it, such as it being fired some time later in the plot. All elements must eventually come into play at some point in the story. Some authors, such as Hemingway, do not agree with this principle.
The principle is recorded in letters by Anton Chekhov several times, with some variation; it was advice for young playwrights.[1][2][3][4]
In recent years, the term has also taken on the meaning of a plot element that is introduced early in a story, whose significance to the plot does not become clear until later.[5][6] This meaning is separate from Chekhov's original intention with the principle, which relates to narrative conservation and necessity, rather than plot significance.
Slow Horses.
Season 1: Sidney is killed off - but Roddy indicates that all evidence of her has been wiped. Is she alive? Will we find out?
Nope.
Season 3: "Operation Footprint" plan is stolen by River, who gives it to his grandfather. OB tosses it into the fireplace. But River has a copy. Will it be used in the future as blackmail?
Nope.
Season 1: Lamb tells Tavener that he knows about her "off book" work with the kidnap plot. Obvious point for future blackmail. Will he use it?
Nope.
There are others, but I can't think of them at the moment.