Following on the grisly murder of King Duncan at the end of Scene ii, Act II, Scene iii opens with the comical protestations of the drunken porter of Inverness. Some critics have famously seen in this episode Shakespeare's concession to his audience benumbed by the sheer horror of the regicide. That may be so, but the mature tragedian is accomplishing a lot more than providing slapstick comic relief. The porter's speech while leading to laughter nevertheless contributes to the larger meaning of the play and is itself a subtle commentary on it. His besotted banter parodies Macbeth's inner torment. This in turn creates a paradox since it is his banter which extends the time between Duncan's murder and the discovery of the body, thereby increasing the tension.
In the porter's reference to himself as Beelzebub's gatekeeper, Act Two's spinechilling metaphor is born in ironic laughter. Macbeth's castle is the vestibule of hell. For the duration of the play the audience will see in Macbeth's power drunk ambition and his wife's fiendish bloodthirstiness the machinations of the Devil.
The porter episode, global in its humour, therefore performs two functions: It lightens the suspense, if even for a moment, but it also expands Shakespeare's dramatic modus operandi, giving the audience an insight into the metaphorical structure of the play.