Perhaps in keeping with Macbeth's paradoxical observation that "fair is foul, and foul is fair," he and his wife, who are dopplegangers, exchange roles back and forth as they prepare for the assassination of King Duncan. First of all, Macbeth writes to Lady Macbeth of his conquests; however, she worries that despite his brutal slaying of Macdonwald, Macbeth is too full of the "milk of human kindness." Therefore, she plans to "pour [her] spirits in [his] ear" and "chastise" him with the "valor" of her speech. Further, she calls upon the spirits to unsex her that she may stop Macbeth from feeling any remorse for the act he is about to commit. She, also, has Macbeth's words only that "King that shalt be" rather than what the witches actually said which was not so definitive: "that shalt be King hereafter." So, she is firmer of purpose than is her husband, who has now has misgivings about slaying his kinsman. In fact, he tells his wife,
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (1.7.34-38)
Thus, when a reluctant Macbeth tells his wife that they will not continue their plan of assassination, Lady Macbeth accuses him of being cowardly and tells him she will judge his love by his acts,
From this time
Such I account thy love. Art thou afeard
To be the same in thine own act and valor
As thou art in desire?
Then, she seeks to bolster Macbeth's courage, and outlines a plan for the assassination. Impressed with her boldness, Macbeth says she should have only male children to inherit this "undaunted mettle." He, then, encourages Lady Macbeth to join him in pretending to be happy before their guests.
False face must hide what the false heart doth know. (1.7.93)
Clearly, in this preparation for Duncan's death, Lady Macbeth takes the leading position, acting as the catalyst for the fulfillment of the prophesy of the witches, encouraging Macbeth, who has some misgivings, while Macbeth praises her for her cunning and courage.