What makes Fortunato fall prey to Montresor in "The Cask of Amontillado"?

To me, there is one thing about Fortunato that we can see clearly and one that we have to guess at.  Both of these seem to have some role in making him fall prey to Montresor.

The clearly evident one is his pride.  Montresor keeps giving him the chance to just go home but his pride won't let him.  Montresor keeps saying that he'll go get Luchesi to see whether the wine is really amontillado.  But Fortunato is too proud to let this happen.

The thing that we have to guess at is why Fortunato doesn't know Montresor hates him.  One guess is that Fortunato is too arrogant and stuck on himself to even think about how others see him.  Maybe he never even thinks about how his actions affect others like Montresor.

What makes Fortunato fall prey to Montresor in "The Cask of Amontillado"?

In Edgar Allan Poe's short story "Cask of Amontillado" Montresor plots to murder Fortunato for an "insult" which is never revealed. Montresor chooses the time of Carnival to carry out the deed. Carnival is the celebration just before the fasting and abstinence of the Christian observance of Lent in anticipation of Easter. Carnival literally means a farewell to meat. 

Amid the chaos of the celebration Montresor lures Fortunato to his palazzo. Montresor knows his servants won't be around because they are out celebrating. He also knows that Fortunato will be a little drunk and that the man will not question the sincerity of Montresor's invitation. Montresor says,

It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my in to smile in his face, and he did not perceive that my to smile now was at the thought of his immolation.

In order to get Fortunato to go into the catacombs where he later meets his demise, Montresor plays on the man's "connoisseurship in wine." Montresor tells Fortunato that he has a "pipe of Amontillado" and he needs his expert opinion. Partly because Fortunato has already been drinking he is quite interested in the fine vintage.

Montresor further entices the man by saying he can always confer with another supposed expert if Fortunato is too busy. Fortunato has disdain for the other expert, further playing into Montresor's hands. Montresor says,

"As you are engaged, I am on my way to Luchresi. If any one has a critical turn it is he. He will tell me --"

"Luchresi 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."

"Whither?"

"To your vaults."

"My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchresi--"

"I have no engagement; --come."

As they descend into the vaults where the wine is stored, Montresor continues to discourage Fortunato by saying it's too cold, the walls are covered in nitre (a salty substance that accumulates in underground caverns) and he fears for Fortunato's health, quite ironic considering what is in store for the man. Montresor does this to disguise his true plan to bring the man to the end of the catacombs, chain him and brick him into the wall, never to be heard from again. 

To this means Montresor also offers more drink to his victim until Fortunato is quite inebriated: "He turned towards me, and looked into my eyes with two filmy orbs that distilled the rheum of intoxication." 

In a final irony Fortunato asks Montresor if he is of the "Masons." The term mason has two meanings. Masons, or freemasons, could be a fraternal order or they could simply be those workers who build with masonry. When asked, Montresor reveals the trowl he plans to use to wall up Fortunato deep in the catacombs below the palazzo.

What makes Fortunato fall prey to Montresor in "The Cask of Amontillado"?

What first allows Montressor to enact revenge against Fortunato for his "thousand injuries" and insult, is that as Montressor states, he never alerted Fortunato to his anger through the issue of a threat, continuing the pretenses of their friendship whilst plotting against him. The crux of what allows Montressor's plan to succeed is, as he states, Fortunato's great weakness is his pride in his "connoisseurship of wine," which allows Montressor to lure him into his company with the promise of a great quantity of Amontillado, a rare vintage. Montressor gets Fortunato drunk and continues to play to Fortunato's pride, eventually luring him into the Montressor house catacombs, where the prized wine is claiming to be stored. Montressor continues to use reverse psychology to get Fortunato farther into the vaults, stating concern for his cough in the damp air, worrying that since he is an important person, people at the carnival will be missing his company. Fortunato, made belligerent through drink and pride, insists on trying the Amontillado, and is eventually led into a niche where Montressor chains him to a wall before entombing him alive.  

What signs showed Fortunato fell for Montresor's amontillado trick in "The Cask of Amontillado"?

Fortunato thought it was all a game because he was drunk. 

It is Carnival, and Fortunato assumes that Montresor is just taking part in the fun. He is too tipsy to evaluate the situation clearly. Montresor shakes Forunato's hand, and seems concerned about his health. His behavior is very disarming. 

"As you are engaged, I am on my way to Luchresi. If any one has a critical turn it is he. He will tell me—"

"Luchresi 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."

"Whither?"

"To your vaults."    

This is reverse psychology. Montresor gets Fortunato into the vault by pretending that he doesn’t want him in the vault. This reverse psychology causes Fortunato to practically beg to see the wine. Montresor protests that Fortunato should not go because he has a cough, but Fortunato insists.

Once underground, Fortunato still thinks they are having fun. He laughs at Montresor's Mason joke and does not even realize he is in danger when he is being bricked up. He still thinks Montresor is joking.

Montresor wasn't joking. Fortunato has no idea what he did to Montresor, but Montresor wants to punish him for whatever perceived injury he did.