Which details contribute to the single effect in "The Cask of Amontillado?"

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.