Answer: With their collaboration and dialogue they are advancing the plot.
Explanation:
Interaction between Montresor and Fortunato is the only thing that is advancing the plot because there is not much happening besides their dialogue. In their dialogue, we can see the hint that is showing us that the letter would be ruined and demised.
Montresor is wanting the revenge and he is trying to set a diabolical scheme because of it which is showing us contradictory in him because he is saying something that doesn't approve his acts. Since the story is told from his perspective, we are more likely on his side and we are empathizing with him.
Montresor is telling to Fortunato that he has something that is mistaken for Amontillado and that is a light Spanish sherry. After that, Fortunato is saying that he has forgotten his family's motto which is "No one attacks me with impunity" and after that, he is quitting the interaction and answering to him when he is saying “For the love of God, Montresor!”.
