In Act I, Scene 5 of Macbeth, Lady Macbeth reads the letter from her warrior husband that informs her of his having been made Thane of Cawdor as well as the witches' prediction that he will be king, along with the news that King Duncan will arrive this night. As she ponders this news, Lady Macbeth worries that the new Thane may not have the temperament for seizing the crown from Duncan.
Glamis thou art, and Cawdor, and shall be
What thou art promised. Yet do I fear thy nature;
It is too full o' th' milk of human kindness
To catch the nearest way....(1.5.15-18)
After reading Macbeth's letter, Lady Macbeth wants her husband to hurry to the castle so she can speak with him about whatever is preventing him from going after the crown, and thereby persuade him against his objections to taking what fate and the spirits seem to want him to possess, anyway. In the meantime, Lady Macbeth calls upon the preternatural world to "unsex" her and fill her "top-full/Of direst cruelty" by taking her "milk for gall"--replacing kindness for bitterness. In this soliloquy of Lady Macbeth's, there is a connection made between masculinity and violence.
When Macbeth arrives, Lady Macbeth convinces him that he must hide what his face reveals after hearing the witches' predictions. "Only look up clear [innocent]" but be like the snake that hides underneath flowers, she tells Macbeth; in other words, give King Duncan no reason to have any suspicion about him. Then, she urges Macbeth to leave all else to her.