There is a sort of interlude between the time Montresor encounters Fortunato up on the streets in the midst of a crowd of revelers and the time he gets his victim down to the bottom of the stairs into his catacombs. It should be noted that Montresor acts in a somewhat zany manner after he and Fortunato are down there all alone. Poe always has a purpose--even a dual purpose--for everything in his story. Montresor wants to keep Fortunato intoxicated, but in order to do so he has to drink with his intended victim. Montresor does not have to drink as much, but he has to act like a "drinking buddy" or else Fortunato is likely to become offended and suspicious. So Montresor's behavior, which includes showing Fortunato the trowel and claiming to be a fellow Mason, can be explained by the fact that he is a little bit drunk himself. The story would lose some verisimilitude if Fortunato were completely drunk and Montresor completely sober. Montresor, after all, owns large quantities of wine. He is obviously a wine-lover, and would certainly drink some of his own wine on this perilous occasion. Furthermore, it would be understandable if Montresor wanted to consume some wine to keep up his courage and calm his nerves. He might also drink some wine to celebrate his achievement in getting Fortunato down off the streets and into his catacombs, where he has him at his mercy. Even if Montresor doesn't manage to get his victim to the place where he intends to chain him to the rock wall, he can murder him any time he wants. He specifies that he is armed with a rapier, while Fortunato must be unarmed because of his tight-fitting costume and because a sheathed sword would be totally inappropriate for the motley costume of a jester.
The Brotherhood is the Masons or Freemasons, a powerful secret brotherhood that still exists. Fortunado asks Fortunado if he is a Freemason. When asked if he is in the Masons, he replies with a gesture the narrator considers rude because he is not in the Masons and does not understand it. Although it does not specify what the grotesque movement is, it seems to be some kind of sign. Fortunado does not believe that the narrator is actually a Mason.
“You are not of the masons.”
“Yes, yes,” I said; “yes, yes.”
“You? Impossible! A mason?”
“A mason,” I replied.
“A sign,” he said, “a sign.”
“It is this,” I answered, producing from beneath the folds of my roquelaire a trowel.
Fortunado does not take the narrator seriously. They leave it at that, and continue. This is a sign that the narrator has some evil plans however. Why else would he bring a trowel? Fortunado does not realize the significance of the trowel. Its presences, and the discussion of the Masons, foreshadows Fortunado’s untimely end. The narrator uses the trowel to brick him up in the crypt.