Short answer: At the banquet in Act III, Scene 4, Macbeth sees the ghost of Banquo.
Renowned Shakespearean critic Harold Bloom observes that Macbeth is a "great killing machine" who possesses less than ordinary intelligence, but has "a power of fantasy so enormous that pragmatically it seems to be Shakespeare's own." Indeed, it is this phantasmagoria connected to Macbeth that produces the tragedy of the mind that unfolds in the play Macbeth. Furthermore, Macbeth's imagination is so powerful that no sooner does he consider a desire or ambition than he perceives it as fait accompli (something already accomplished). This trait certainly awakened in Elizabethan audiences those parts of the imaginations in themselves that they found so frightening much as horror films do to audiences today.
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