Answer: It is advancing the plot because they are having a dialogue
Step-by-step explanation:
In the plot, there isn't much happening besides their interaction and that is the only thing that is advancing the plot.
They are having a dialogue that is referring to the demised letter. Montresor is contradictory because he is wanting the revenge and his acts don't approve that what is he saying.
Although, the story is told from his perspective and that is why we are empathizing with him and his acts. Montresor is talking with Fortunato and saying that light Spanish sherry can be mistaken for Amontillado and because of that, Fortunato is telling him about the forgotten motto of Montresor's family which is that no one attacks someone with impunity.
The dialogue ends with ''For the love of God, Montresor''.