Lady Macbeth's opening words introduce a new level of emotional intensity. Fear of failure has been replaced with fear of discovery, and even though she describes herself as drunk with boldness and on fire with passion, she is just as easily alarmed as her husband is by the tiniest noises and movements. Her swift changes of thought and speech foreshadow the language of her final lapse into madness in the sleepwalking scene (Act V, Scene 1), when she relives these same moments.
Yet, despite all this, Lady Macbeth appears to be sufficiently hardened to the deed to be able to make several horribly ironic comments, including the observation that she would have committed the murder herself, had she not been put off the idea by the resemblance of the sleeping king to her own father. Note the similarity of this line — by which she seems to excuse something lacking in herself — with her earlier taunt to Macbeth that she would have dashed out the brains of her own child had she sworn to do so. The fact is that what Lady Macbeth would do her husband has actually done. The total reversal of roles that she anticipated cannot now occur because, despite his stricken conscience, Macbeth has done what she could never do.
