What is Luchesi's role in the short story "The Cask of Amontillado"?
Fortunato does not necessarily believe that Luchesi is an "ignoramus." In fact, he may believe that Luchesi is at least as knowledgeable about wines as himself. He is only telling Montresor that Luchesi is an ignoramus because that is what he wants him to think. Throughout the story Fortunato is apprehensive that Montresor will consult Luchesi, and Fortunato doesn't want the other man even to know about the existence of the Amontillado.
Montresor says he has bought one pipe (126 gallons) at a bargain price. There must still be a whole shipload of Amontillado in the harbor. Fortunato is not anxious to sample Montresor's wine in order to enjoy a glass or two. Amontillado is a good wine, but it is not rare; he could easily find a bottle in a liquor store if he was so fond of it. He is not anxious to sample it to show off his connoisseurship of wines. He is not anxious to go to all that trouble at that time of night in order to do a favor for a friend. Fortunato wants to make a big profit by buying as many casks of the Amontillado as he can and then selling it in bottles. He visualizes a whole shipload containing nothing but huge oaken casks of the finest Spanish sherry. But he has to sample it to make sure it is the real Amontillado.
Montresor has shown he is a shrewd judge of human character. He knows that Fortunato is already planning to tell him the wine he supposedly bought is only an ordinary sherry and then find the ship and buy up the entire cargo. Fortunato is rich and can become even richer. He will not only make a profit but consider his trickery an "excellent jest." Tricks like this are among the thousand injuries he had inflicted on Montresor over the years. Fortunato wears a jester's costume because he thinks of himself as a jester and trickster. Montresor knows he couldn't trust him to tell the truth about the Amontillado--if it existed. He would taste it, wrinkle his nose, shake his head, and say it was just ordinary Spanish sherry. Unfortunately for Fortunato, there is no Amontillado.
Luchesi is a minor character in Edgar Allan Poe's short story, "The Cask of Amontillado." Luchesi never actually appears in the story, but he serves an important purpose as the bait to which Montresor is able to lure Fortunato into the catacombs. When Montresor tells Fortunato that he has a rare bottle of Amontillado for which he would like an expert's opinion, Montresor suggests that he will visit Luchesi instead of wasting Fortunato's time.
“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—”
“Luchesi cannot tell Amontillado from Sherry.”
“And yet some fools will have it that his taste is a match for your own.”
“Come, let us go...”
“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—”
“Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.”
Of course, Montresor has counted upon this reaction from Fortunato, knowing that he will not pass up the chance to sample a rare vintage. Before taking the final steps to what will be Fortunato's final resting place, Fortunato has one more word about Luchesi:
"He (Luchesi) is an ignoramus."
Montresor says at the beginning of the story, "THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge." So the answer to your first question is "Fortunato INSULTED him". Poe never tells us exactly HOW or what it is that Fortunato says.
Luchesi is the impetus Fortunato needs to go into the catacombs without questioning Montresor. Fortunato thinks himself a wine "expert". Because of this, Montresor knows that he can use Luchesi as "bait"--the promise of a full pipe of amontillado at Carnival is such that Fortunato doesn't want Luchesi to have an opportunity to sample it. It is with blind greed that Fortunato follows Montresor into the catacombs.