Fortunato insulted the narrator so now the narrator wants to get revenge on Fortunato.
Keep going straight then you'll find it!!!
The narrator already new about Fortunato unsluting him but no one knew about his plan. After Fortunato insulted the narrator they met at a festival and talked about wine. While they were talking at the festival the narrator was coming up with a plan to capture Fortunato.
Oh no I can't get out! I'm trapped!
Fortunato had no idea about the narrator's revenge so when Fortunato went to try the narrators alcohol he had no intention of anything happening to him. The narrator's plan to capture Fortunato was a big secret to everyone.
The narrator guided Fortunato through the underground cask where he told Fortunato the alcohol was. The narrator had a plan to tell him where to go and then Fortunato end up getting trapped in his trap in the far back.
As fortunato was walking through to underground cask looking for the wine he gets trapped and is not able to get out.
Once the narrator gets Fortunato in the trap he starts to have doubts and his heart starts to race a little more than usual.