Lady Macbeth and Macbeth have three soliloquies between them before the murder of Duncan. The dramatic purpose of these is twofold: first, it lets the audience know the main characters' thoughts in the pivotal and suspenseful time before the murder of a monarch. Second, the soliloquies build up a mood and atmosphere of chilly and relentless terror, depicting a world taken over by the dark side.
In act I, scene v, Lady Macbeth receives the letter from her husband telling her the witches' prophecies. She is as ambitious as Macbeth is and wants the crown as much as he does but fears he will be too virtuous to do what it takes to realize their joint lust for power. Therefore, she calls on dark spirits while talking herself into a state of pitilessness and hardness so that she can goad her husband into performing the worst of deeds, tyrannicide. In lines such as the following, she build up the mood of foreboding and horror that will continue to grow, asking for a heart:
Of direst cruelty. Make thick my...
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