It is advancing the plot because they are having a dialogue
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''.
With their collaboration and dialogue they are advancing the plot.
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!”.
my explanation is the word explanation is spelled EXPLANATION
answer: life tends to copy things that already exists rather than taking old ides and making it into something new