Montresor explains to "You, who so well know the nature of my soul" that he had decided to murder Fortunato, but to wait until conditions were perfect for accomplishing his objective. In the meantime, he was establishing a future alibi by acting as if Fortunato were his best friend. He not only convinced Fortunato that they were friends, but he seems never to have mentioned his name without calling him "My friend Fortunato," "My dear friend Fortunato," or something else along those lines. Montresor had gotten so conditioned to calling Fortunato his friend, to acting friendly towards him, and even to think of him as his good friend, that he calls him "My friend," "My good friend," and "My poor friend" innumerable times throughout his tale. For example, Fortunato has a coughing fit, and Montresor states:
My poor friend found it impossible to reply for many minutes.
When Fortunato turns up missing, there will be a long and extensive inquiry. But nobody will suspect Montresor of complicity because everybody will feel sure that the two men were the very best of friends. In fact, no will will show more concern about Fortunato's disappearance than Montresor himself. It could place demands on his acting ability for years.
This is one of the ways in which Montresor uses his acting ability to get his revenge. He prepares for the future. He tells his confidant (or confidante) that:
A wrong is unredressed when retribution overtakes its redresser.
He wants to be sure he will not only not get caught but will not even be a suspect.
Montresor uses his acting ability when he first encounters the drunken Fortunato celebrating the carnival on the street. Montresor not only lies about having received a pipe of Amontillado, but he pretends to be in great haste to have some authority verify its authenticity. He says he is on his way to Luchesi, since he wasn't able to find Fortunato. The act makes Fortunato assume that Montresor got a bargain on the wine and is anxious to buy a few more pipes while it is still being offered as a bargain price, but can't do so until he is sure he has gotten the real Amontillado. It is the idea of a "bargain" that motivates Fortunato to go with Montresor to taste his nonexistent wine. And Fortunato doesn't want Montresor consulting Luchesi, presumably another expert, because Luchesi would want to get in on the "bargain" himself.
Montresor puts on an act all the time he is leading Fortunato down the stairs into his wine vault and through the corridors. One of the ways in which he uses his acting ability to motivate Fortunato to continue following him to his destination and his doom is to use reverse psychology. He keeps suggesting that they ought to turn back because Fortunato has a cold and the catacombs are terribly unhealthy. For example:
“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”
This reverse psychology not only brings out Fortunato's contrariness, but it seems to offer assurance that there could be not danger in continuing in the same direction. (It is interesting that Montresor says, "You are a man to be missed." This is a another way of reassuring Fortunato of his safety. If he disappeared he would be missed, and there would be many people out looking for him. Montresor knows Fortunato will be missed. That is why he is taking such precautions about exacting his terrible revenge.)
Montresor consistently acts humble, deferential, and fawning. This is intentionally disarming. Even at the end, when Fortunato is chained to the rock wall and Montresor is walling him up, Montresor continues to act friendly, considerately, and obsequiously.
"Pass your hand,” I said, “over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my power.”
With his intricate lies and his acting ability, Montresor has his victim completely fooled.