He met Fortunato while he was alone in the street one evening. He spoke to him amiably - a sign he drunk much. Montressor played along, acting as if he's pleased to see him.
"Fortunato! How are you?"
"Montressor! Good evening, my friend"
"Fortunato had hurt me a thousand times and I had suffered quietly. But then I learned that he had laughed at my proud name, Montresor, the name of an old and honored family. I promised myself that I would make him pay for this — that I would have revenge."
"I would make him pay, yes; but I would act only with the greatest care. I must not suffer as a result of taking my revenge."
He continued to smile at Fortunato's face as to not raise suspiscion. The fool believed it! Montressor then remembers Fortunato's fatal flaw - wine. Montressor bought the best one; revenge, alas!