Dramatic irony is a literary device by which an author makes his readers privy to a character’s situation while keeping the character in the dark. He may employ this device to elicit a particular emotional response in readers as well as to keep them hooked until the character himself discovers the final outcome of his actual situation.
Dramatic irony constitutes the framework of Poe’s "The Cask of Amontillado." As readers, we do know the sinister designs of Montresor when he coaxes Fortunato to the catacombs under his house, but Fortunato doesn’t. He doesn’t even have the slightest idea that with each step he nears his death.
At the very outset of the story we are informed about the secret macabre plan of Montresor. He has been nurturing a deep-seated grudge against Fortunato for some insult and so he wants to avenge himself:
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.
We pity Fortunato when he keeps on drinking more and more of the wine Montresor offers him, and descends deeper into the eerie cellar. We know that this is a part of Montresor’s plan. Our knowledge about Montresor’s macabre designs keeps us on toes until the moment when finally Fortunato learns about his tragic situation. It’s too late by then. He can’t escape his death any more. The dramatic irony sharpens our sense of fear, pity and helplessness in the story.