Edgar Allan Poe's short story, "The Cast of Amontillado," is the story of Montresor's revenge against Fortunato. It is clear from the text that Fortunato and Montresor have a history together, and that it's been a difficult relationship, at least from Montresor's perspective.
In the opening line, Montresor, who narrates the story, states: "The thousand injuries of Fortunato I had borne as best I could, but when he ventured upon insult, I vowed revenge." The reader is never told what injuries Fortunato has inflicted upon Montresor, but one can use other evidence from the text to determine the nature of insults Fortunato has likely made against Montresor.
Later in the story, while the pair is in the catacombs, Fortunato makes a gesture that Montresor does not return. Fortunato makes the statement that Montresor does not comprehend, and this shows that he is not of the brotherhood. When Montresor replies yes, insinuating that he is a mason, Fortunato replies "You? Impossible! A mason?" This would have been insulting to Montresor. The Freemasons are a rather secretive society and an exclusive one. Anticipants, which is what a person wanting to join the organization is called, have to be recommended by someone in order to join. Often, one's family history is taken into account when a person is being considered. When Fortunato expresses disbelief that Montresor could be a part of the brotherhood, he is insinuating that he is not worthy to be a Freemason, either by his own virtues or because of his family's status, or both. Montresor insists that he is a mason. When Fortunato demands proof, he shows him his trowel. Fortunato is speaking of the secret society, and Montresor is talking about the profession in which one uses mortar and bricks or stone to build things. He is about to become that type of mason in order to exact his revenge on Fortunato.
According to the Montresor, the two men must have been friends prior to Montresor's plot of revenge. Montresor says: "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." Montresor repeatedly refers to him as his friend throughout the text, both to his face and as part of the narration. This fact of friendship gives the reader questions as to the psychological health of the narrator, for what true friend carries out a plot of destruction as cruel as the one Montresor executes?
If they were good friends, why would a mere insult cause Montresor to bury his friend alive? Aside from the questions of the narrator's psychological health, Montresor gives us one other clue as to his actions. Fortunato asks him about his family's coat of arms. Montresor describes it as an azure field with a huge human foot crushing the head of a serpent whose fangs are in its heel. Then Fortunato asks what the motto is. Montresor replies: "Nemo me impune lacessit." This is Latin for no one insults with impunity. Ironically, Fortunato does not see the connection between the insults he has hurled at Montresor and the family motto. Montresor has carefully hidden his true feelings from Fortunato.
Another piece of textual evidence that points to the history of the two men is the dialogue between them at carnival about the Amontillado. Montresor knew Fortunato well enough to know that he considered himself a wine connoisseur and that if he told Fortunato that he'd go to Luchesi instead, Fortunato's interest would be piqued. He knew that Fortunato's pride would compel him to follow Montresor, and he uses the "carrot" of Luchesi to lure him deep into the catacombs to his doom.