How does the relationship shift between Montresor and Fortunato?

Edgar Allen Poe's "The Cask of Amontillado" is all about who has power over who (at least in Montresor's mind, that is). At the beginning of the famous short story, we learn from Montresor, the narrator, that he feels Fortunato has crossed him many times in the past, but recently Fortunato has crossed the line and done something that Montresor cannot forgive.

The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.

Before the story starts, Fortunato has the upper hand in their relationship being the insulter rather than the insulted, but it is clear that Montresor intends to change that (and he very well does). 
When they first meet up in the story, Montresor makes Fortunato think that they're still on good terms, complimenting him on his knowledge of wine, and never letting a hint drop that he's still seething about the unspecified "insult". As the story moves on, we realize that Montresor's revenge is trapping Fortunato in his catacombs to die. Montresor lets the false niceties fall away, and Fortunato can see his true nature: calculating, cruel, and possibly insane. 
So, their relationship shifts from that of false friendship (which might never have been real friendship in the first place, given how much Montresor hates Fortunato), to that of a revenger who has total power over his victim.