As they walk into the catacombs, Fortunado begins coughing.
Fortunado even says:
“Enough,” he said; “the cough's a mere nothing; it will not kill me. I shall not die of a cough.”
This is an example of foreshadowing, because he does die at the end of the story.
This is also ironic because the narrator gets him to drink, because he’s so concerned about the cough. He is tricking Fortunado into trusting him and letting down his guard.
As they continue, they discuss the vaults and the wine that is supposedly found there. The most significant part of the conversation is not actual conversation, but Montressor’s thoughts. He is planning to get revenge on Fortunado, even while pretending they are the best of friends.