Although Montresor says he has already bought the Amontillado, he pretends to be in a hurry to have an expert assure him that it is genuine. He tells Fortunato:
You were not to be found, and I was fearful of losing a bargain.”
It is the idea that whole pipes of Amontillado are available at a bargain price that interests Fortunato. He naturally thinks Montresor would like to buy more of the wine if he could be sure of its quality. The only way a cargo could have come to Venice was by ship. Fortunato does not need to sample Montresor's wine; he could go to the harbor and easily find a Spanish ship newly arrived from Barcelona and laden with big barrels of Amontillado sherry. That is why Montresor has fine-tuned his story to include Luchesi. Fortunato doesn't want this other man to hear about the Amontillado because then he would be competing with him in bargaining for the Spanish wine and driving up the price. That is why Fortunato has to go to Montresor's palazzo. He probably has no intention of telling Montresor the truth--and Montresor knows it because he has had plenty of experience with this man! Assuming this Amontillado is genuine, Fortunato, the jester, will taste it and tell Montresor it is only ordinary sherry. Thus he will eliminate both Luchesi and Montresor as competitors in bidding for the remainder of the Amontillado aboard the ship--or he would do so if the wine or the ship existed at all.
Fortunato is proud of his connoisseurship, but he is not merely interested in showing it off to Montresor. He wants to taste the wine to make sure it is genuine, and he has to go to Montresor's palazzo to sample it because he doesn't want Montresor going to Luchesi.
Montresor has another reason for pretending that he has been looking all over for Fortunato. Montresor wants to make sure that Fortunato is not expected anywhere that night. He emphasizes at the beginning of his tale that he wants to commit his murder with "impunity." He needs to lure his victim off the streets and down into his catacombs without being recognized as his companion. He wants to leave a cold trail. He doesn't want anybody who might have been expecting Fortunato that night to send people out looking for him. This is why he says:
And then, not getting the information he wants:
"I perceive you have an engagement. Luchesi—”
To which Fortunato replies:
“I have no engagement;—come.”
This is what Montresor wants to hear. But how can he pretend to believe that Fortunato has an "engagement," that there is someone expecting him? Only because he pretends to have been searching all over for him without being able to find him.
Later when Fortunato finds himself in chains he will try to make Montresor think he was expected at his home by his wife, relatives, and a number of guests.
"But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest? Let us be gone.”
By saying "awaiting us," Fortunato is trying to plant a seed of doubt in Montresor's mind. He is suggesting that many people had seen them together and had recognized Montresor and that these people assumed the two men were on their way to Fortunato's palazzo, which is where many of those same people were heading themselves to partake in the big festivities being held there.
Fortunato is trying to frighten Montresor into releasing him and at the same time offering him an excuse for doing so by pretending to believe that this entrapment is only an elaborate practical joke. But Montresor has had the foresight to establish that Fortunato has no engagement that night and will not be missed until tomorrow morning at the earliest. By that time everybody will be recovering from hangovers. If they remember seeing Fortunato in his jester's costume, they will not remember the shadowy figure who was with him wearing a black cloak and a black mask. It doesn't matter how many people saw Fortunato as long as nobody recognized Montresor.
We know almost from the beginning that the narrator of "The Cask of Amontillado" by Edgar Allan Poe is not quite sane. He talks to us as if we were somehow his conspirators, and he is proud of his intricate and well designed plan to get revenge. Although Montresor is relatively insane, he is certainly bright enough to be able to use his enemy’s weakness against him.
We do not know what Fortunato did to insult Montresor—if he did anything at all—but we do know that the two of them were colleagues in the field of wine. Montresor says:
He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine…. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; I was skilful in the Italian vintages myself, and bought largely whenever I could.
So Montresor’s plan hinges upon Fortunato’s belief in his wine connoisseurship. After Montresor ensures that his house is empty of servants, he seeks out Fortunato and begins to bait him. He does this by telling Fortunato that he has bought some amontillado.
This is the middle of Carnival, a several-week celebration in which the wine is flowing, and of course Fortunato does not believe Montresor. Now Montresor puts his plan in action and flatters Fortunato by suggesting that Montresor should never have bought the amontillado without consulting the greatest expert, Fortunato. Even more, when Fortunato is still kind of dumbstruck at the idea of such a fine drink being available during Carnival, Montresor sets the hook by mentioning Luchesi.
Luchesi is an arch-rival, and Fortunato considers himself to be far superior to the lowly Luchesi. Montresor uses Fortunato’s arrogance and pride in his wine expertise to lure him into his cellar. Each time Fortunato hesitates, Montresor simply mentions Luchesi’s name and Fortunato’s pride propels him forward. Each time Fortunato stops, Montresor keeps him moving by casually suggesting (though Fortunato sees it as threatening) that he will simply ask Luchesi to certify the amontialldo. No one has as fine a palate as Fortunato, according to Fortunato, and he insists on being the only one to see this precious cask of amontillado.
Montresor knows his enemy well, and his plan works. Fortunato is easily lured into the cellar simply because he arrogantly believes that no one could do a better job of certifying the amontillado than he can. Montresor knows exactly what he is doing, and Fortunato’s pride costs him his life.
The main clue to the character of Fortunato is not the unreliable narrator, Montresor and his narrative, but the man's ironic donning of a harlequin costume that he wears for the Carnival. For, this apparel signals Fortunato's fatuity, and he demonstrates his complacent stupidity at every turn of the catacombs' structure.
The gait of my friend was unsteady, and the bells upon his cap jingled as he strode...."Enough!" he said, "the cough is a mere nothing, it will not kill me. I shall not die of a cough."
His costume also is indicative of Fortunato's self-deception as he obviously does not consider himself foolish at all, or he would not have donned such a costume; neither would he have allowed himself to be so easily seduced by his delusion of being a connoisseur of wines, whose appraisal of a wine is better than that of many, especially Luchesi.