Stephano and Trinculo consistently offer comic relief throughout the play. Their time on the island consists of a bumbling series of events, as the result of chaos and confusion and resulting in even more. A drunk Stephano, for example, stumbles upon Caliban and Trinculo and mistakes them for a monster, and it isn’t long before all three are cooking up a half-baked scheme to kill Prospero and supplant Stephano as king of the island. Not only does the plan fall apart before it can gain any traction, but it only leads to further captivity, confusion, and total disengagement from anything close to plausible.
Trinculo, during the episode in which Stephano takes him and Caliban to be a monster, only ends up hiding alongside him due to his own curiosity and unfazed lack of critical thinking. The persistent inebriation of all three characters adds an ongoing swerve of chaos to the calculated chessboard maneuvers orchestrated by Prospero. Stephano and Trinculo also provide social commentary of the civilized world. Their boorish behavior indicates that they believe they deserve to inherit the island just as Prospero did when he arrived years previously. Like Prospero, Stephano and Trinculo immediately position themselves as more important and worthy of power than Caliban, whom they see as a subject in their kingdom, illustrating the entitled and manipulative nature of “civilized” society.
By the play’s end, they are given no extreme punishment by Prospero, but are chastised and sent away. There does not seem to be any explicit growth from them, nor any major ripple effects from their influence. If anything, their drunken and manipulative exploits serve to humanize Caliban.