a character does. Obviously, the inebriated Porter is not aware of the fact that the humble and gracious KingDuncan was murdered in the house mere moments prior to the knocks sounding at the door. For all he knows, he just spent a typical and peaceful night guarding a lovely castle. He is just a drunkard looking to pass the time while he guards the door and waits for morning (this is probably a rather boring job). However, we know what has just transpired; a hell on earth has actually been created by Macbeth's heinous actions, and this creates some tension as a result of the irony.

Many questions have been posted in eNotes about the drunken porter scene in Macbeth(Act 2, Scene 3). Many of the answers deal with the concept of comic relief. It seems likely that Shakespeare inserted comic scenes in some of his tragedies because his audiences liked to laugh. The plays were entertainment, diversion. One might compare the comic interludes to the shows that used to be offered in movie theters. There would be a serious drama plus a cartoon featuring characters like Bugs Bunny or Tom and Jerry, and there might be a short subject with comedians like the Three Stooges. Since Shakespeare was only presenting a single feature (so to speak). he might have been inclined to insert comedy as a sort of bonus to please the members of his diverse audience who wanted that sort of thing.

No doubt Shakespeare had at least one member of his company who was especially good at making people laugh. He couldn't just assign any actor to a role like that of the drunken Porter. Good comedians are rare. If he had a talented comic he would be thinkinig of using him often, and it well might be that the same actor who played the drunken Porter in Macbeth also played the gravedigger in Hamlet. Critics who have questioned the purpose of the drunken Porter scene have also questioned the purpose of the gravedigger scene. Both may have been devised purely to utilize the talents of a particular member of Shakespeare's company.

Furthermore, the same comedian might have played the part of the Fool in King Lear, of Touchstone in As You Like It, and many other parts, including the clownish peasant who brings the two poison adders to Cleopatra in Antony and Cleopatra. In Julius Caesar there is a part for a comic who calls himself a cobbler and is leading a group of Commoners in the very first scene. One might visualize this actor as someone who looked and acted like Bert Lahr in TheWizard of Oz--a natural comedian who could make audiences laugh just with facial expressions and tones of voice.

Shakepeare may not have been so much concerned about "comic relief" as he was about simply providing a little comedy and making use of the special talents of one of the members of his company.

I think you are right in indicating that this important scene plays an important function in terms of giving comic relief after a very fraught scene with Macbeth actually doing the deed and killing Duncan. However, consider its other purpose: this scene really helps to maintain tension by the knocking and by our feeling that the discovery of the murder is merely being delayed by the rambling talk. You might want to consider too how the horror of the murder is somehow intensified by the coarse vulgarity of the Porter; it seems, in his comic bad taste, to be a gruesome attempt to cover up the truth. However, note too, how when we read his comments alongside what Lennox says about the night, his words give a contemporary and a universal significance to Macbeth's crime.

It is important therefore not to simply underestimate the Porter as mere comic relief when Shakespeare seems to use him for so many other reasons as well.

Your question addresses the dramatic (or theatrical) purpose of the scene rather than the thematic purpose. Many debate/discuss the possibilities of the scene involving the Porter who holds the keys to the castle door as a metaphor for the gates of Hell, and the Porter does mention "hell-gate" and "Beelzebub" (another name for the devil).  But these observations don't answer the question of how the scene operates in a theatrical or dramatic sense, merely a thematic one.

The scene is certainly meant to be funny, and the actor playing the Porter would have been one of the "clowns" in Shakespeare's company of actors, but the scene is not simply one of comic relief.  It operates on a much more cohesive level dramatically than this.

In Act II, scene ii, knocking is introduced.  The audience can't be sure what this knocking is, and, at first, neither can Macbeth and Lady Macbeth.  The tension and suspense of this scene are running very high, since the scene, rather than being a presentation of Duncan's murder, is a scene all about the possibility that Macbeth and Lady Macbeth might get caught.

They both begin the scene hearing things: an owl, the bellman, the crickets.  They also might be hearing voices:

Lady Macbeth

Did not you speak?

Macbeth

When?

Lady Macbeth

Now.

Macbeth

As I descended?

Lady Macbeth

Aye.

Macbeth

Hark!

This creating of suspense through real and imagined offstage sounds is a tactic still used today and one you can find in many horror and suspense movies.

So, this scene is all about the tension of whether they will be caught or not.  When Lady Macbeth leaves to return the daggers to the murder scene, Macbeth hears knocking.  And at this moment, he isn't sure what it is, but Lady Macbeth enters and says, "I hear a knocking / At the south entry," identifying it as someone beating to enter the castle.

Again, it sends the message of urgency to the audience.  Is it someone who knows that Duncan has been murdered?  It certainly seems that way, because of the focus that Shakespeare has placed in the scene upon the possibility of Lady Macbeth and Macbeth being discovered.

The knocking builds and continues into scene iii, and, though the Porter is funny, dramatically, the tension of the scene is not relieved because the audience still really wants to know who is out there and whether they might expose Macbeth as the murderer.

Interestingly, once the door is finally opened to admit Macduff (his first entrance into the play), he is already, because of the association made between the knocking and the murder, planted by Shakespeare in the audience's mind as someone who knows what Macbeth has done.  And he, in fact, will be the one thane who refuses to attend Macbeth's banquet in Act III.

So, though the scene is a funny one, it actually serves dramatically to heighten the audience's anticipation concerning the question of whether Macbeth and Lady Macbeth will get caught.  And this is all because the knocking is established as part of the suspense of Act II, scene ii.

Shakespeare would often insert a bit of comedy into his plays after a very dramatic scene, as in the case of Duncan's murder. The porter's speech lends this element as he pokes fun at various people, especially those who are drunk. In addition, the porter is pretending as though he is opening the gate to hell. Consider what has just taken place and how relevant that is.

The purpose of this scene is known as "comic relief". The audience has just witnessed the killing of Duncan and the drama surrounding it and this comic scene gives the audience a chance to catch their breaths and recover from all the drama that they have just witnessed. In addition, the scene also establishes the evil atmosphere around Macbeth's castle. When the porter curses, he does not use God's name but instead uses the "name of Beelzebub", or the devil. When he hears a knock on the door, he predicts the knock is from one of three people who are all associated with hell, a farmer who committed suicide, someone who committed treason, or a thief. He even refers to people walking "to the “everlasting bonfire.” These references help to symbolize the fact that Macbeth is "raising hell" in Scotland.

Knock, knock! Who's there, in the other devil'sname? Faith, here's an equivocator, that couldswear in both the scales against either scale;who committed treason enough for God's sake,yet could not equivocate to heaven: O, comein, equivocator.

Many will tell you that the Porter scene is comic relief, but they are wrong.  His language and actions may seem comical by today's audience, but to Elizabethans (at least the educated ones), his performance is grotesque, and his language is not unlike the devil's.

First of all, the most heinous crime in all of Elizabethan England or in any Medieval honor culture is to kill a king.  Regicide is akin to killing God.  Duncan's murder is not shown on stage for a reason.

Next, the knocking on the door parallels the sound imagery from the scene before, with the bell.  Macduff is the hero of the play.  He was born not of woman (like virgin birth).  He sacrifices his family for the salvation of the country.  He is a Christ-figure, and his knocking is a foreshadowing of the salvation to come.

I think the Porter's speech is grotesque, rather than comic.  Some critics think he's the devil.  All would agree that Macbeth's castle has become a hell, so who else would guard its gates?

The Porter also talks of equivocations (half-truths, paradoxes), like the witches.  The Devil is the great equivocator: he seduces with lies and false promises.  He tempted Christ with "Man shall not live with bread alone" and Eve with "You shall not surely die."

Equivocal morality frightened Elizabethans, almost as much as regicide.  To blur the lines of good and evil was a great fear.  We have all seen what moral relativism has done to the power of the church and the slackening of traditional moral values.  It was indeed a death knell to Christian theology and opened the door of the occult philosophy that was prevalent in the day.