How does Shakespeare use Banquo's ghost to present Macbeth in Act 3, Scene 4?

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In Act 3, Scene 4, Macbeth's response to seeing Banquo's ghost gives the audience a good picture of his mental state at this point in the play.  When he first sees the ghost, he says to his guests, "Which of you have done this?" (3.4.59).  He is immediately suspicious of those men to whom he should feel closest: they are his lords, his advisers, those closest to his crown.  However, this -- his first response -- indicates that he is guarded even against them.  His suspicious nature is confirmed later in the scene when he tells his wife that "There's not a one of them but in his house / I keep a servant fee'd" (3.4.163-164).  In other words, he has a spy in each of his closest advisers' homes!  This also helps to convey his growing paranoia: his conscience is so guilty at this point that he feels himself to be suspected by everyone around him.

We also get to see how badly Macbeth wants to keep up appearances.  He tries to seem confident and controlled and powerful to all, and so he lies, saying, "I have a strange infirmity, which is nothing / To those that know me" (3.4.104-105).  In other words, he throws responsibility on the lords, insisting that he only has a minor, though odd, health concern, and they should consider it nothing if they want to remain close to him.

Further, his inability to control his immediate response to seeing the ghost also shows that he is not really as brave as he'd like to think.  This fact is conveyed to readers in other ways as well: it doesn't take a big man to order the killings of Banquo and his young son, and, later, Lady Macduff and her children.  He has to have others do his dirty work because he was too shaken by his one experience with murder to repeat it.  Lady Macbeth points out his cowardice in this scene as well: she asks him, "What, quite unmanned in folly?" (3.4.88), and says that Macbeth is "Shame itself!" (3.4.79).  Even when Macbeth tries to confront the ghost, he does so quite emotionally and fearfully, and it is only when the ghost exits that he says, "Why so, being gone, / I am a man again" (3.4.130-131).  The ghost "unmans" him, and this feels pretty shameful to both his wife and himself.

The appearance of Banquo's ghost shows us Macbeth's guilty conscience, his lack of courage, his desire to maintain appearances, and his growing suspicion and paranoia.  He is beginning to come unhinged.