He is gonna act like everything cool but in his mind he is planning revenge.
“The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge.”
“As you are engaged, I am on my way to Luchesi. If any one has a critical turn, it is he. He will tell me—"
“Luchesi cannot tell Amontillado from Sherry."
He is tricking Fortunato into coming with him to start his revenge.
This is when he leads his so called friend down to the wine cellar.
This when he sees that his friend is taking the effects of his revenge but ticks him into wanting to stay.
“we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—"
They found the wine and are about to leave
He left his friend in the cellar and finally got his revenge