What effect does Banquo's ghost have on Macbeth?
Macbeth is struck with fear and guilt when he sees Banquo's ghost. This is a crucial turning point in the play, as it is when we see Macbeth expressing his internal guilt for the first time. It remains possible, even given the significant role of the supernatural, that the ghost is not supernatural, but rather a manifestation of Macbeth's inner turmoil. None of the other guests at the banquet, including Lady Macbeth, can see the apparition, and it thus seems very odd when Macbeth begins shouting at it:
Avaunt, and quit my sight! Let the earth hide thee!
Thy bones are marrowless, thy blood is cold;
Thou hast no speculation in those eyes
Which thou dost glare with.
As Macbeth continues to shout at the invisible ghost, his wife asks their guests to leave. When they do leave, he talks openly about the sense of guilt and fear that the ghost has aroused in him:
It will have blood: they say blood will have blood.
Stones have been known to move and trees to speak;
Augres and understood relations have
By maggot pies and cloughs and rooks brought forth
The secret'st man of blood.
The ghost, then, reveals the fear, guilt, and paranoia that Macbeth's deeds have caused. He worries that Macduff, who did not attend the banquet, may be conspiring against him, and decides that the only thing to do is to consult the witches for advice.
Banquo's ghost appears at a banquet held by Macbeth and Lady Macbeth in Act III, Scene IV. When Macbeth first sees the ghost, he thinks that it is a trick played on him by his Lords. He asks them, for instance, who has done it ("Which of you have done this?").
Macbeth's reaction then changes as he confronts the ghost. He claims that he is innocent of Banquo's murder:
Thou canst not say I did it. Never shake
Thy gory locks at me.
Macbeth is also desperate for others to see the ghost, especially his wife. But Lady Macbeth claims that the ghost is nothing more than a hallucination and says it will quickly pass. This only increases Macbeth's sense of desperation, as we see when he urges her to look at the ghost:
Prithee, see there! Behold! Look! Lo! How say you?
Finally, after the ghost vanishes and reappears, Macbeth's reaction is one of anger. He shouts at the ghost to get out of his sight, for example, and claims that he is no longer afraid of it. In fact, he tells the ghost to come back to life and to challenge him to a duel. The ghost does indeed disappear, leaving Macbeth to breathe a considerable sigh of relief.