The single effect in "The Cask of Amontillado" is deception, from the characters name, Fortunato, or good luck, to the drink that Montresor gives his doomed guest, De Grave, to the use of the idea that Montresor is going down into the catacombs to sample an elite wine, everything is based on deception.
Montresor carefully hides his true intention, revenge, beneath a respectful and cordial demeanor. The fact that Montresor pretends to worry about Fortunato's cough as they descend into the catacombs, even suggesting that they turn back is all deception.
Cast against the backdrop of the carnival, a deceiving setting, the plot of this short story is absolute horror.
It is important first to understand what is meant by "the single effect." This is a phrase that was coined by Poe himself. He meant that a short story's elements and details should work together so seamlessly that this "unique or single effect" is created.
In a novel, because of length, plot, characters, theme, symbolism, and other literary elements can be revealed through a slower process including commentary and description. However, a short story should be more economical and therefore elements are revealed in a more straightforward way, through action. By revealing details about mood, setting, plot, and characters through action and concise narration, the author creates a story with a single effect.
As Poe was the pioneer of the single effect, it is safe to say that all of the details contribute to the single effect, as Poe might argue that for a short story to be successful all details must contribute to the single effect.