Literary Devices in Macbeth
Martlets, or martins, are small birds related to swallows. Banquo indicates that this summer bird is attracted to Macbeth's castle and has chosen to make its nest on it. Banquo describes a quiet and idyllic summer day, which contrasts later with the storm that comes when Macbeth commits the foul deed.
Macbeth uses this metaphor to compare his ambition to a horse rider who falls when he jumps higher than he is able. In these lines Macbeth recognizes that he has no other reason to kill Duncan than his own ambition. This blatant recognition of his dangerous ambition makes Macbeth's eventual murder of Duncan ironic.
This brief exchange reveals how much Macbeth loves Duncan: he cannot sit in the same room with the man he is planning to murder. Lady Macbeth's reply indicates that Duncan obviously loves Macbeth because he has always been an honest and honorable man. Macbeth is in an unfamiliar role that he is uncomfortable playing. Shakespeare uses moments like this to maintain a degree of sympathy for Macbeth, since the tragedy of the play is his downfall.
Notice here how Macbeth somewhat uses the royal "we" in this passage and refers to himself in the plural. In doing so, he speaks as if he were already crowned king.
By "infirm" Lady Macbeth uses the antiquated understanding of the word: not firm, or unstable. In a previous scene, Lady Macbeth told Macbeth that they could not fail if he shored up his courage. However, with this line, Lady Macbeth recognizes that he was not as firm as she wanted him to be, and thus they are in danger of being caught.
Shakespeare wants both husband and wife smeared with blood. Naturally Lady Macbeth will return with bloody hands and some blood on her garment after smearing the faces of two men with Duncan's blood. In doing this, Shakespeare creates the illusion that the audience has witnessed a horrible murder without showing it on stage.
Knocking is a motif throughout Macbeth. After Macbeth and his wife complete their hideous murder, they hear a knock within that causes them to immediately fear and begins their slow descent into guilty madness. The Porter imagines Hell's Gate and unwittingly invokes two symbols. First, Macbeth's castle begins to resemble hell as he has committed a hideous crime. Second, the Porter reminds the audience that bad deeds, such as murder and suicide, are met with punishment in hell. This foreshadows not only Macbeth and Lady Macbeth's coming end, but the promise of punishment in the afterlife for their ambition. It is unclear whether or not this line was the basis for the modern Knock, Knock joke.
Compare this short speech with Macbeth's earlier one in lines 99-104. Whereas earlier he appeared genuine, here the word choice and poor imagery help to illustrate Macbeth's hypocrisy.
Even though they're talking about the weather, Macbeth's statement here is a perfect example of understatement. For him, the weather is of little concern considering the kind of "rough night" Macbeth has just had.
The Porter makes a pun here. The word lie, as in "lie down," sounds the same as the word lye which means "urine." The humor is in the idea that too much drink makes someone fall asleep and possibly wet themselves.
As Macbeth and his wife leave the courtyard, the scene doesn't change. The porter, slowly wakened by the knocking from his drunken sleep, makes his way to the door. His speech helps move the time along as Macbeth cleans his hands and tries to pretend that he was asleep while Duncan was murdered.
Macbeth continues his extended metaphor to provide more meaning for his comparison: Some dogs have special qualities given to them by nature, and Macbeth hopes that these murderers also have gifts that will allow them to best carry out the murder.
Lady Macbeth uses this speech to calm Macbeth's guilty conscious. However, her flippant response to Macbeth's worries, essentially there's no changing what has happened so forget about it, comes back to haunt her when she begins to feel pangs of guilt. Her "what's done is done" later transforms into "what's done cannot be undone," as Lady Macbeth becomes overrun with guilt.
In a way very characteristic of his style, Shakespeare plays on the word "peace" by putting two instances close together with multiple meanings. The first instance refers to the satisfaction of Macbeth's desire for power that he had hoped to gain by killing Duncan; and the latter instance refers to the peace of death.
Shakespeare has the murder of Banquo shown onstage as opposed to not showing Macbeth murder Duncan earlier in the play. The choice to show this death likely revolves around how this scene represents a turning point, and Shakespeare also wanted the audience to know that Banquo is truly dead--especially considering the visions that Macbeth has after he learns what happened.
One of the possible reasons why Shakespeare includes a third murderer in this scene as a way to ensure that dialogue takes place. Without the Third Murderer, it's possible that the other two would wait in silence for Banquo and Fleance. However, with the third, the audience is able to experience the scene in a more complete and engaging way.
While saying his own name here might be a little awkward, the presence of "our" indicates that this phrase is another use of the "royal we," and Macbeth freely resorts to this egotistical language having heard what he considers to be good news from the apparitions.
As the witches brew a potion in the cauldron, they repeat this chant as a kind of evil spell. Interestingly, Shakespeare uses tetrameter (four beats per line) instead of iambic pentameter for the witches spell. This incantation, the cauldron, and the evil atmosphere all credit Shakespeare with the creation of the Western conception of a typical witch.