On what is Fortunato an expert?
Fortunato, as Montresor concedes, is an expert judge of wine. But Montresor does not really want Fortunato to judge the Amontillado because it doesn't really exist. Montresor uses Fortunato's pride in his connoisseurship to lure him into the catacombs, where he can chain him to the rock wall and leave him to die an agonizing death. The fact that Fortunato is an expert judge of wine makes it plausible that Montresor should seek him out to judge a cask of Amontillado he supposedly acquired at a bargain price.
Even if the Amontillado had really existed, and even though Montresor regards Fortunato as an expert judge, Montresor knows exactly what Fortunato would be thinking and planning. Fortunato would make a big show of smelling and tasting the wine and finally shake his head and tell Montresor it was only ordinary sherry. Fortunato knows that Montresor, a poor man, would only have bought a cask containing 125 gallons of Amontillado because it was a "bargain" and he hoped to resell it at a profit. Here are the most significant parts of Montresor's story:
“I have my doubts,” I replied; “and I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain....As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”
This is calculated to make Fortunato believe that Montresor plans to buy more of the Amontillado if only he can be assured it is genuine. If genuine, it is a bargain; if not, it is not worth buying more. Fortunato knows that Luchesi would also be very interested in what appears to be a newly arrived cargo of Amontillado being sold off at bargain prices, probably because it is the height of the carnival season and nobody, including Fortunato, is doing business. He has to go to Montresor's palazzo that very night to prevent him from going to Luchesi, who is probably rich enough to buy up the whole cargo, as Fortunato intends to do himself. The fact that he is an expert does not mean that he is trustworthy. He is thinking of discouraging Montresor from buying any more of the nonexistent Amontillado by judging it to be bogus. Then he will have the field all to himself. He can get an even greater bargain because he is rich enough to take the entire cargo.
Why doesn't he question Montresor about this Amontillado? Where did he get it? How much did he pay? He doesn't want to show too much interest. That would only make Montresor guess his true intentions. (Montresor doesn't have to guess. He knows his man. He has been injured by him a thousand times, mostly in business dealings of this sort, no doubt. He knows Fortunato would try to cheat him and then laugh about it as an excellent jest.) Fortunato wants Montresor to think that he is just doing him a big favor by tasting his wine--but the discriminating reader should understand that Fortunato is not the type of man to go out of his way to do such a favor when he is having fun carousing in the streets, when he is inadequately dressed to go into a damp underground wine cellar, and when he has a bad cold. His only interest is in making a large amount of money, and he has to keep Luchesi from hearing about the bargain-priced Amontillado. He can't put Montresor off for the night because Montresor is obviously in a hurry to buy more of the Amontillado before word gets around that it is available at a bargain price. If Fortunato doesn't go to Montresor's palazzo that very night, then Montresor will go straight to Luchesi.
Fortunato doesn't really need to taste Montresor's (nonexistent) Amontillado. He could easily find the ship bearing the cargo and taste the wine directly from one or two of the casks on board (if such a ship and such a cargo existed), But if he makes up an excuse for not accompanying Montresor to his palazzo, Montresor will go to Luchesi. Montresor has thought out his entrapment scheme very thoroughly. He twice pretends to believe that Fortunato has an "engagement." He gets the information he wants on his second try.
“My friend, no; I will not impose upon your good nature. I perceive you have anengagement. Luchesi—”
“I have no engagement;—come.”
Fortunato is not expected at home or anywhere else. He won't be missed until sometime tomorrow at the earliest. Montresor wants to leave a cold trail. Everybody is drunk. They may remember seeing Fortunato that night, but they won't remember in which direction he was walking or a shadowy companion wearing a black cloak and a black mask.
Fortunato prides himself on being a wine connoisseur. However, while he is deserving of respect on this connoisseurship in wine, Fortunato, Montresor observes, is a "quack" in the knowledge of the arts. This pride of Fortunato and the shortcomings of the man are both upon which Montresor works his scheme to lure Fortunato into his family catacombs on the pretext of tasting the newly-acquired Amontillado.
That Fortunato is rather foolish is indicated by the harlequin costume which he wears during the Carnival. With great irony, Montresor greets him and repeatedly shakes his hand, "My dear Fortunato, you are luckily met. How remarkably well you are looking today!" Then, Montresor plays upon the intense interest in wine of Fortunato by saying that he has just received a large cask of "what passes for Amontillado (a variety of sherry), and I have my doubts....And I must satisfy them." As if this will not entice Fortunato to accompany him, Montresor acts as though he does not need the connoiseur,
"As you are engaged. I am on my way to Luchesi. If anyone has a critical turn it is he. He will tell me--"
Pride dominates Fortunato, who interjects, "Luchesi cannot tell Amontillado from sherry." He insists that he accompany Montresor and falls into the subtle trap.