Banquo's knowledge of the witches' prophecy makes him both a potential ally and a potential threat to Macbeth's plotting. For now, Macbeth seems distrustful of Banquo and pretends to have hardly thought of the witches, but Macbeth's desire to discuss the prophecies at some future time suggests that he may have some sort of conspiratorial plans in mind. The appearance of Fleance, Banquo's son, serves as a reminder of the witches' prediction that Banquo's children will sit on the throne of Scotland. We realize that if Macbeth succeeds in the murder of Duncan, he will be driven to still more violence before his crown is secure, and Fleance will be in immediate and mortal danger.
Act 2 is singularly concerned with the murder of Duncan. But Shakespeare here relies on a technique that he uses throughout Macbeth to help sustain the play's incredibly rapid tempo of development: elision. We see the scenes leading up to the murder and the scenes immediately following it, but the deed itself does not appear onstage. Duncan's bedchamber becomes a sort of hidden sanctum into which the characters disappear and from which they emerge powerfully changed. This technique of not allowing us to see the actual murder, which persists throughout Macbeth, may have been borrowed from the classical Greek tragedies of Aeschylus and Sophocles. In these plays, violent acts abound but are kept offstage, made to seem more terrible by the power of suggestion. The effect on Lady Macbeth of her trip into Duncan's bedroom is particularly striking. She claims that she would have killed Duncan herself except that he resembled her father sleeping. This is the first time Lady Macbeth shows herself to be at all vulnerable. Her comparison of Duncan to her father suggests that despite her desire for power and her harsh chastisement of Macbeth, she sees her king as an authority figure to whom she must be loyal.
Macbeth's trepidation about the murder is echoed by several portentous sounds and visions, the famous hallucinatory dagger being the most striking. The dagger is the first in a series of guilt-inspired hallucinations that Macbeth and his wife experience. The murder is also marked by the ringing of the bell and the knocking at the gate, both of which have fascinated audiences. The knocking occurs four times with a sort of ritualistic regularity. It conveys the heavy sense of the inevitable, as if the gates must eventually open to admit doom. The knocking seems particularly ironic after we realize that Macduff, who kills Macbeth at the end of the play, is its source. Macbeth's eventual death does indeed stand embodied at the gate.
The motif of blood, established in the accounts of Macbeth's and Banquo's battlefield exploits, recurs here in Macbeth's anguished sense that there is blood on his hands that cannot be washed clean. For now, Lady Macbeth remains the voice of calculating reason, as she tells him that the blood can be washed away with a little water. But, as Lady Macbeth eventually realizes, the guilt that the blood symbolizes needs more than water to be cleansed away. Her hallucinations later in the play, in which she washes her hands obsessively, lend irony to her insistence here that "[a] little water clears us of this deed" (2.2.65).
