Montresor's plot for revenge hinges on a critical fact; Fortunato believes himself to be an expert on the taste and authenticity of wine. Because of this, Fortunato will be not only willing but insistent on verifying Montresor's fictional cask of Amontillado wine, in order to prove himself superior to Luchresi, a locally-accepted expert.
He had a weak point... He prided himself on his connoisseurship in wine. Few Italians have the true virtuoso spirit.
"The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchresi, he cannot distinguish Sherry from Amontillado."
(Poe, "The Cask of Amontillado," xroads.virginia.edu)
Montresor needs to get Fortunato into the catacombs of his own free will; note that each time Montresor asks Fortunato to return, Fortunato insists on continuing to find the cask. Because of this, Montresor doesn't need to do anything drastic, such as drugging Fortunato or knocking him out; Fortunato willingly goes to his doom, assured that...
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