The fact that Montresor had the materials for bricking Fortunado up shows that he prepared.
Montresor makes it clear that Fortunado insulted him many times, over time.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
He also says that he planned this revenge carefully so he would not get caught. You can see evidence of this extreme forethought and planning in the fact that he waited for carnival season and made sure Fortunado was drunk.
Montresor produces a trowel from “beneath the folds” of his cloak. He makes a joke out of it, pretending to be a Mason, so Fortunado does not realize what is going on. By the time they get deeper down into the catacombs, it is too late.
… I busied myself among the pile of bones of which I have before spoken. Throwing them aside, I soon uncovered a quantity of building stone and mortar. With these materials and with the aid of my trowel, I began vigorously to wall up the entrance of the niche.
Montresor had these supplies carefully hidden, and had the trowel with him. He had planned the murder to the last detail. He was interested not only in getting revenge, but also in getting away with it. For Montresor, getting away with the murder was just as important as the murder itself.