Act 2 Scene 3 of William Shakespeare’s Macbeth is often known as ‘the Porter scene’. The Porter, the one comic turn in an otherwise overwhelmingly dark and violent play, dominates the scene, as well as making reference to the most momentous event of 1605, the shock of which would have been fresh in the minds of Shakespeare’s original audience in 1606.
In order to provide an analysis of the Porter scene, we’re going to go through the scene, stage by stage – until the Porter pretty much disappears from view. What follows, then, is part-summary, part analysis of this central scene in Macbeth – the scene which follows the murder of King Duncan at the hands of Macbeth himself.
[Knocking within. Enter a PORTER]
Before we go any further, let’s set the (literal) scene: at the Macbeths’ castle, where the King of Scotland, Duncan, has been staying as their guest, Macbeth (goaded by his wife and by the prophecy of the Witches) has just murdered Duncan in his bedchamber, with a view to taking the throne for himself. A knocking can be heard at the front gate of the castle: Macduff, a Scottish nobleman, is the one doing the knocking.
What is the purpose of the Porter’s comic interlude, though? A practical reason has been proposed: the actor playing Macbeth spoke of the ‘blood’ on his hands in the previous scene with Lady Macbeth, so he would need to go and clean his hands and get changed out of his bloody costume before coming back on stage.
Indeed, Samuel Taylor Coleridge thought the dialogue so unlike Shakespeare that it must have been written by someone else, ‘for the mob’ (i.e., to please the crowd). Alexander Pope also thought that one of Shakespeare’s fellow players had written it, and he consequently gave it short shrift. However, there are artistic reasons for this scene, too, as we will see. And the scene is not just comic relief – indeed, it is important that it is not even ‘comic relief’ in the true sense at all. But we’ll come to that …
PORTER
Here’s a knocking indeed! If a man were porter of hell-gate, he should have old turning the key.
The Porter’s first words in the scene allude to the medieval Mystery plays (that is, a play designed to teach the audience good Christian 
morality through dramatising biblical stories), about ‘The Harrowing of Hell’; there are several extant versions of these plays, which do indeed take place at the Gates of Hell, and some of them even have a porter at Hell-gate, one of whom even calls to Beelzebub (as Shakespeare’s Porter does).
There’s a degree of dramatic irony in the Porter’s words: unbeknown to him, thanks to the actions of the Macbeths the Porter is at the gates of ‘Hell’, in a way, because of the evil deed that has taken place at the castle. After all, Lady Macbeth had even called upon the devil’s agents or spirits to attend her so that she and her husband could kill the king (‘Come to my woman’s breasts, / And take my milk for gall, you murdering ministers, / Wherever in your sightless substances / You wait on nature’s mischief!’). It is as if the Macbeths’ Inverness castle has become Hell itself.
The Porter’s use of ‘old’ (‘he should have old turning the key’) is slightly odd, but he means essentially ‘he would be turning the key so frequently it would soon get old’.
[Knocking within]
Knock, knock, knock! Who’s there, i’ the name of Beelzebub? Here’s a farmer, that hanged himself on the expectation of plenty: come in time; have napkins enow about you; here you’ll sweat for’t.
Did you know Shakespeare invented the ‘Knock knock’ joke? Well, this appears to be the first appearance of the formation ‘knock, knock! Who’s there’. And although what follows isn’t a ‘joke’ as we’d understand it, the Porter’s reference to the farmer is a piece of comic relief (and a sort of forerunner to the sort of references Sam Weller makes his own in Dickens’s The Pickwick Papers). In essence, the (imaginary) farmer the Porter refers to hanged himself because he got greedy and stored up grain, only for the price of grain to plummet, thus plunging him into penury.
[Knocking within]
Knock, knock! Who’s there, in the other devil’s name? Faith, here’s an equivocator, that could swear in both the scales against either scale; who committed treason enough for God’s sake, yet could not equivocate to heaven: O, come in, equivocator.
Another ‘knock, knock’ joke from the Porter – and a very topical one. ‘Equivocation’ is the idea of lying when under oath (e.g. when being interrogated by the authorities), in order to serve some greater cause. It predates Shakespeare’s play, but in early 1606 it was ‘in the news’ because of the high-profile trial and execution of a Jesuit priest, Father Henry Garnet (who was known, oddly enough, as ‘Farmer’: recall the Porter’s previous mention of a farmer who hanged himself).
Garnet had advocated that Jesuits, if interrogated about whether they had harboured a Catholic priest in their house, should ‘equivocate’: lying without lying outright, if you will. For instance, one might say ‘no priest lies in my house’ (because they are standing up and hiding behind a cabinet at that precise moment, for instance, so not technically lying down).
The Porter’s reference to ‘treason’ alongside ‘equivocator’ makes it highly likely that these lines from the Porter scene, if not the whole scene itself (and maybe the whole play) were written in 1606.
[Knocking within]
Knock, knock, knock! Who’s there? Faith, here’s an English tailor come hither, for stealing out of a French hose: come in, tailor; here you may roast your goose.
Yet another ‘knock, knock! Who’s there?’ line from the Porter. This time, an English tailor has arrived at the Porter’s imaginary ‘hell’ (there are lots of old jokes against tailors like this). A ‘goose’, as well as being a tasty bird, is a tailor’s pressing-iron: so ‘roast your goose’ is a joke, because of the fires of hell.
[Knocking within]
Knock, knock; never at quiet! What are you? But this place is too cold for hell. I’ll devil-porter it no further: I had thought to have let in some of all professions that go the primrose way to the everlasting bonfire.
The Porter cleverly reminds us where we are meant to be: up in Inverness, in the far north of Scotland, where it is indeed ‘too cold for hell’. The ‘primrose way’ (compare Hamlet’s ‘primrose path to dalliance’) is a flowery, beautiful, pleasant path – but it leads to ‘the everlasting bonfire’ of hell. The Porter decides to leave off his play-acting that he’s the porter at the gates of hell.
[Knocking within]
Anon, anon! I pray you, remember the porter.
[Opens the gate]
[Enter MACDUFF and LENNOX]
MACDUFF
Was it so late, friend, ere you went to bed,
That you do lie so late?
PORTER
’Faith sir, we were carousing till the second cock: and drink, sir, is a great provoker of three things.
The Porter is often played as hungover, clutching his head as if suffering from a headache: he was up late drinking (‘carousing’) till three o’clock in the morning (‘the second cock’, i.e. when the second cockerel crowed).
MACDUFF
What three things does drink especially provoke?
PORTER
Marry, sir, nose-painting, sleep, and urine. Lechery, sir, it provokes, and unprovokes; it provokes the desire, but it takes away the performance: therefore, much drink may be said to be an equivocator with lechery: it makes him, and it mars him; it sets him on, and it takes him off; it persuades him, and disheartens him; makes him stand to, and not stand to; in conclusion, equivocates him in a sleep, and, giving him the lie, leaves him.
Like so much in the Porter scene, this comic exchange about how getting drunk makes men lustful, but removes their physical ability to perform in bed, has more in common with the central themes of the play than we might first realise. As Kenneth Muir notes in his excellent introduction to “Macbeth” (Arden Shakespeare: Second Series)
MACDUFF
I believe drink gave thee the lie last night.
Macduff is not a character known for his jokes in the rest of Macbeth, but here he permits himself to join in with the Porter’s punning: ‘gave thee the lie’ means ‘laid you up on your back’. In other words, he’s saying the Porter appears to have got so drunk that he passed out from it.
PORTER
That it did, sir, i’ the very throat on me: but I requited him for his lie; and, I think, being too strong for him, though he took up my legs sometime, yet I made a shift to cast him.
The Porter responds with a wrestling analogy: although drink laid him up on his back, he was stronger than it, and when drink ‘took up my legs’ (i.e., picked him up by the legs), the Porter responded by throwing drink off (possibly with a suggestion of throwing up).
MACDUFF
Is thy master stirring?
[Enter MACBETH]
Our knocking has awaked him; here he comes.
The Porter’s section of this scene has given the actor playing Macbeth enough time to change out of his bloody clothes and wipe his hands, and now the main action of the play can resume. But the Porter scene, as this analysis shows, is more than just comic relief: in some ways, in reminding us constantly of the dark event that has just occurred at the castle (even if the Porter has no knowledge of Duncan’s murder), Shakespeare intensifies the horror of the murder, much as when we hear people make blithe reference to something horrific it shocks us all the more for being so offhand.
The persistent knocking in the scene is also possessed of dramatic power. Thomas De Quincey wrote a whole essay on it.