The choice of narration in Poe's "The Cask of Amontillado" is very important to the reader's complete understanding of the characters.
While it is true that the first person narration of Montersor could have told the entire story of the murderous plot, the use of dialogue allows readers to see the extent of Montresor's anger and weakening mentality and the drunkenness of Fortunato.
Montresor's laughter that resounds off the catacomb walls and Fortunato's screams both would not be as chilling if presented by a narrator alone. The dialogue adds to the fear and worry the reader feels for Fortunato, the wickedness of Montresor, and the overall sickness illustrated throughout the tale.
Montersor could have allowed emotion to seep into a reader's mind in the same way it does when Fortunato states, "FOR THE LOVE OF GOD, MONTRESOR!" and Monresor replies, "Yes," I said, "for the love of God!" The feelings the reader is left with is chilling.