The letter, read alone on stage by Lady Macbeth, reiterates the Witches' prophecy of Act I. Significantly, in his letter, Macbeth says nothing of their prophecy to Banquo; perhaps he is already afraid of its implications. Equally significantly, he sets up Lady Macbeth as his "dearest partner of greatness." She will indeed become his partner in crime, but much more than that: Apart from the fatal blow itself, she will be responsible for controlling Macbeth's passions and — to an extent — his actions.
Immediately after she finishes the letter, Lady Macbeth's mind goes to work. Her words "shalt be" uncannily reflect those of the Witches' prophecy. At this point, Lady Macbeth herself has virtually become an agent of Fate, just like the Weird Sisters. But immediately her thoughts turn to possible failings in her husband. He is "too full of the milk of human kindness" to commit murder; he would be great, he would have a high position, he would wrongly win that position, but in each case, some other aspect of his character would not. In this case, she says, there is only one solution. She must "pour [her] spirits in thine ear." Any member of Shakespeare's audience who had seen his play Hamlet four years previously would be more than aware of the significance of this line, for in that play the good King Claudius is murdered by poison administered through the ear. The scene is rapidly becoming darker.
