In most stories the protagonist has a motivation to achieve something, and the reader wants to see if he will succeed or fail in achieving it. This usually involves keeping the reader in the protagonist's point of view from beginning to end. A good example would be Jack London's "To Build a Fire." The protagonist wants to get to a camp where he will be safe and warm. He keeps running into problems and in his case fails to achieve his objective; but we remain interested to the end because we want to find out what happens. In most stories the reader is kept interested because he wants to find out what happens.
In "The Cask of Amontillado" we know that Montresor wants to murder Fortunato. We don't know whether he will be successful or exactly how he intends to murder his enemy. We want to find out what happens. Montresor has numerous problems to cope with even before he gets Fortunato underground. He has to lure Fortunato off the crowded street without being recognized himself. Fortunato is wearing a gaudy costume and even a cap with ringing bells. This turns out to be an advantage because Fortunato attracts all the attention and Montresor, in a black cloak and black mask, is not noticed. He is like a shadow. Then when Montresor gets Fortunato down into his wine vaults and the catacombs he has to keep his victim drunk, and in at least one instance he has to distract him from asking questions about the Amontillado--questions such as, "Where are we going?" "Why is it so far from the bottom of the steps?" "Why are we taking so long?"
“The pipe,” he said.
“It is farther on,” said I; “but observe the white web-work which gleams from these cavern walls.”
Montresor gets Fortunato interested in the nitre gleaming all around him. He talks about other things as well, all intended to keep Fortunato distracted during their long journey to the place of execution, where Fortunato expects to find a huge pipe of gourmet Amontillado sherry. Another way in which Montresor keeps Fortunato distracted and confused is by urging him to go back.
“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—”
Montresor has many problems to cope with, but finally we see how he has accomplished his purpose, which was to commit a perfect crime without any risk of being suspected as the perpetrator. He has Fortunato chained inside the narrow niche and is building a wall to hide him forever. The story quickly ends because the protagonist has achieved his goal and the reader's curiosity is satisfied.