After receiving her husband's letter about the witches' favorable prophecies, Lady Macbeth becomes overwhelmed with ambition and begins to formulate a plan to assassinate King Duncan. When Macbeth arrives home, she cautions him to appear pleasant and friendly in front of the king in order to conceal his malevolent intentions. When King Duncan arrives at Macbeth's castle in act one, scene six, Lady Macbeth behaves like a gracious, benevolent host toward the king. She acts friendly and respectful towards Duncan and even refers to herself as his servant. Lady Macbeth's pleasant behavior is convincing, and King Duncan doesn't have any suspicions about her evil motives.

Later that night, Lady Macbeth reveals her evil personality and cruel intentions by criticizing her husband for second-guessing his decision to murder King Duncan. Lady Macbeth proceeds to insult Macbeth's masculinity before assuring him that they will get away with the crime. Overall, Lady Macbeth reveals her capacity for dissembling by appearing to be a gracious, courteous host in front of King Duncan while simultaneously plotting his brutal death behind his back. Behind Duncan's back, Lady Macbeth is portrayed as a callous, determined woman, who is ruthless, cruel, and resolute.

Upon hearing of the witches' prophecies, Lady Macbeth has already begun to fantasize about conspiring to take Duncan's crown. In Act One, Scene Five, she gives the famous "unsex me" soliloquy in which she asks the spirits to make her more masculine and/or supernatural so that she will have the courage to carry out a strategy that will gain Macbeth the crown. To avoid arousing any suspicion, Lady Macbeth is extremely welcoming when Duncan arrives. She claims that she and Macbeth are his loyal servants. But behind the scenes, she and Macbeth are conspiring to kill him. During the feast, Lady Macbeth feels compelled to criticize Macbeth when he starts to have misgivings about killing Duncan. She tries to reawaken the greed ("desire") that Macbeth felt after hearing the prophecy that he might become king:

                                Art thou afeard

To be the same in thine own act and valour

As thou art in desire? (I.vii.l39-41)