Why is the diction in "The Cask of Amontillado" formal?

The diction in the story is not merely formal but exaggeratedly so. The way Montresor speaks is overly correct, and his word choice seems to indicate a person who takes pride in his large vocabulary. Poe seems to be satirizing this type of vain, affected person. The diction serves as an indicator of education, refinement, and erudition. In this it matches other indicators of the narrator's status.

However the content of his speech, along with the overall message of his narrative, is at variance with the outward form. The narrator's goals of revenge, his obsession with being slighted, reveal a person whose more basic instincts are dominant. No matter how much he may have benefited from his superior education, his unstable mind will take over and push him into disastrous action.

Why is the diction in "The Cask of Amontillado" formal?

The first reason is due to Edgar A. Poe's writing style, which was formal in nature. Poe was a master of words and had an extensive vocabulary, and he made use of it in his writing. His style is instantly recognizable, just as Hemingway's or Faulkner's would be to most people who are well read. The other reason is because this diction Poe uses would be indicative of the character of the narrator. The narrator was a well-to-do man who was upper-class, so his diction would be formal, unlike someone who was from the lower classes. The narrator is also very calculating and careful, so one would suspect his diction would also be this way.

In "The Cask of Amontillado," which diction is essential to the story?

In the opening paragraph there is a semantic field of words connoting or explicitly denoting revenge and retribution: "revenge . . . threat . . . avenged . . . punish . . . punish . . . retribution . . . redresser . . . avenger." This ensures that the impetus for the story is very clear. We know straight away that this will be a revenge story. We also know, because of the frequency of these words in the paragraph, just how determined the narrator is, and, by implication, how hurt he must have also been by the wrongdoing that he says has been done to him.

When the narrator accompanies Fortunato back to his vault, he "put(s) on a mask of black silk and draw(s) a roquelaire closely about (his) person." The "mask" and the "roquelaire" (cloak) form his disguise, and this imagery of disguise reflects the narrator's true intentions, namely to deceive and take vengeance upon the witless Fortunato.

The descriptions of the setting are also significant in adding to the macabre, gothic atmosphere of the story. The narrator takes his victim, Fortunato, into the damp catacombs, where there are "long walls of piled skeletons" and many piles of bones. There is also "white web-work which gleams from the cavern walls." The "web" here might allude to a spider's web, which symbolizes how the narrator has trapped Fortunato in this place, just like a spider traps a fly in its web.

Also significant is the imagery describing the narrator's family crest, which depicts a "foot crush(ing) a serpent." The serpent symbolizes Fortunato, who the narrator sees as treacherous and villainous, just like the biblical snake who deceived Adam and Eve. The foot in the crest symbolizes the narrator, who is about to kill Fortunato, as the foot is about to kill the snake. The narrator's family crest also has a motto, which is "Nemo me impune lacessit." This translates to "no-one provokes me with impunity." This is, of course, significant because it echoes exactly what the narrator is thinking. He has been provoked by Fortunato (although we never find out what that provocation was) and he is determined that Fortunato won't get away with it.

In "The Cask of Amontillado," which diction is essential to the story?

In "The Cask of Amontillado," the narrator Montresor's well-developed vocabulary and formal diction helps to give us clues about his intelligence, likely one of the sources of his immense pride, as well as his cunning. After his enemy, Fortunato, insults him, he "vowed revenge." He says,

At length I would be avenged; this was a point definitively settled—but the very definitiveness with which it was resolved, precluded the idea of risk. I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redressed.

This passage clearly exhibits Montresor's elevated diction.  He could say, more conversationally, that he had definitely decided that it was time for revenge and that this revenge would have to take place without the possibility of his own guilt being discovered. If he is punished for the revenge, then it doesn't really count as revenge. Instead, he uses words like "precluded," "impunity," "retribution," and "unredressed": atypical words for the average Joe. We know that it was some insult that served as the last straw for Montresor, and because he seems so intelligent, it was perhaps an insult to his intelligence that finally pushed him over the edge. Such an insult would wound his pride and likely compel him to take action. Further, the level of intellect indicated by such language usage gives us a peek into Montresor's mind: he is clearly meticulous and attentive to detail, qualities he will need to pull off and get away with the perfect crime. Anyone with such a well-developed and precise vocabulary would have to be. Thus, his diction helps to foreshadow his eventual success in this endeavor.