What is Luchresi's role in the story? And also, what preparations had Montressor made for his revenge?
Montresor uses Luchesi as a kind of spur to get Fortunato to come to Montresor's house to see the supposed Amontillado he's purchased. Montresor has identified pride as Fortunato's weakness, and he calls up that pride in a couple of ways. First, he tells Fortunato that he purchased a large quantity of Amontillado, a rare kind of Spanish wine, because he seems to know that Fortunato will not believe it. Moreover, he knows that Fortunato will not want to pass up an opportunity to prove him wrong. Even before they return to Montresor's home, Fortunato says, "'Amontillado? A pipe? Impossible! And in the middle of the carnival!'"
Second, Montresor knows that if he says he'll get another local wine connoisseur to verify the wine, this will further compel Fortunato because he will not want to believe that this other expert is as discerning and capable as he. When Montresor says that he'll find Luchesi since Fortunato is obviously busy, Fortunato says, "'Amontillado! You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.'" Fortunato believes that his expertise is superior to Luchesi's as well as Montresor's and he will not pass up this opportunity to exercise it, just as Montresor planned.
In terms of Montresor's other preparations, he's brought with him a "mask of black silk" as well as a "roquelaire" (or cloak) that he draws around himself. This way, he cannot be identified by anyone who happens to see him and Fortunato together. He also hides the trowel, a small spade one would use to lay brick, beneath his cloak. Moreover, he was sure to tell his servants that he would "not return until the morning, and had given them explicit orders not to stir from the house," knowing full well that this would "insure their immediate disappearance, one and all, as soon as [his] back was turned." Therefore, no one from Montresor's own household would be able to say that saw Montresor with Fortunato either.
Luchesi is the trump card, the ace up the sleeve that Montresor uses to dupe Fortunato into the catacombs beneath his home. Luchesi is a second rate rival of Fortunato's in wine expertise. Montresor has bought a keg of Amontillado, a rare and pricey wine, which Luchesi assures him is the real thing. Appealing to Fortunato's pride in wine connoisseurship, Montresor asks for a second opinion. Fortunato's pride is stung by Montresor asking Luchesi's opinion first.
The rocks and mortar were already in the cellar, as were the chains. Montresor carries a trowel hidden in his cloak. Getting Fortunato drunk, exposing him to the niter on the walls, which made him cough, renders his resistance weak. The surprise of the attack seals Fortunato's fate.