If we only look at what is said in their conversation, then we can infer that Montresor is a loyal friend and Fortunato is proud and stubborn. With Montresor's additional narration, we infer that he is a cold, calculating, evil man who will do anything to achieve vengeance. But without his narration, just focusing on the conversation, he comes across as a thoughtful friend. He mentions the amontillado because he knows Fortunato would love to prove his mastery of wines. Montresor politely says that since Fortunato is busy, he will ask Luchesi if the amontillado is legitimate. Fortunato shows his pride and insults Luchesi. "Luchesi cannot tell amontillado from Sherry."
When Fortunato begins to cough, Montresor replies that they should go back. On the surface of this conversation, it appears that Montresor is thinking only of his friend's health. But in reading the narration, we can infer that Montersor is practicing a bit of reverse psychology. He knows that Fortunato will not want Luchesi to make the determination and he affirms his resolve to check the amontillado himself.
Being friendly and appearing to be concerned with Fortunato's health has fooled Fortunato completely. This is why Montresor's strategy works. His behavior and words in conversation do not reveal his true nature. He appears to be a loyal friend, thus hiding his murderous intentions. In conversation, Fortunato looks bad because he behaves like an elitist connoisseur and is too stubborn to consider his own health.