When acted well, this scene is the highlight of the play. We know that Banquo's ghost is sitting in the chair reserved for Macbeth, but only Macbeth and the audience see the ghost--thus, we have dramatic irony. The scene is eerily humorous. Macbeth cannot control his reaction upon seeing the ghost and even shakes off Lady Macbeth's chiding that he is acting cowardly:
The times has been
That when the brains were out, the man would die,
And there an end; but now they rise again
With twenty mortal murders on their crowns,
And push us from our stools.
When he attempts to pull himself together and lament Banquo's absence at the banquet, the ghost appears again. The audience sees the ghost before Macbeth does, and the anticipation of his reaction is enormous. Shakespeare does not fail to deliver. Macbeth is clearly shaken:
Avaunt! and quit my sight! Let the earth hide thee!
Thy bones are marrowless, thy blood is cold . . .
This scene shows us the enormous changes that Macbeth has undergone. With Duncan's murder, Macbeth instantly felt remorse and guilt. He knew he would never sleep well again and wished that the murder could be undone. However, with Banquo's murder, Macbeth's guilt becomes more subconscious. He has suppressed his scruples over murdering his friend Banquo, even jovially praising the murderers who killed him. Yet, we see that Macbeth is not an entirely evil man. He is still wracked by his conscience, albeit suppressed. The ghost is Shakespeare's dramatic way of showing Macbeth's inability to murder without suffering pangs of conscience.
It is after this scene that Macbeth completely distances himself from Lady Macbeth and turns toward the witches for help.
In Act 3 Scene 4 of Shakespeare's Macbeth, Macbeth holds a banquet for the others in the kingdom and while at dinner, he thinks that he sees the ghost of Banquo. Macbeth addresses the ghost, but obviously the other characters do not see the apparition so the scene is marked by dramatic irony. Further, Macbeth's guilt over having Banquo murdered is what prompts the ghost, but no one else at the banquet knows that Banquo has been killed and that Macbeth is responsible for the murder. This scene parallels the scene in Act 2 Scene 1when Macbeth conjures the dagger before he kills King Duncan. Macbeth's guilt causes him to see these images and both the dagger and Banquo's ghost are symbols of Macbeth's inner conflict.
The banquet scene, as it is often called, marks the climax of the play. Banquetting with all its connotations of a natural fest where the natural process of nourishment goes on and its contrast in the human carousals that signal chaos e.g. the nocturnal fest in Inverness before Duncan's murder.
The scene marks the beginning of Macbeth's end. It contains his inadvertent public confession of his crimes, terrified by the ghost of Banquo.
It is the last time we see Macbeth and Lady Macbeth together. Her failure in controlling the situation looks forward to her end as it were.
The trajectory of the scene is from perfectly and artificially crafted hierarchical order to chaotic dissolution.
Banquo's ghost, like the air-borne dagger earlier, continues the subjective supernatural trope.
Banquo's ghost is the essential point of irony. He does keep his promise of attending Mabeth's coronation banquet by arriving even after being killed by Macbeth's men. Every appearance of his is also marked by verbal irony. The moment, Macbeth hypocritically laments Banquo's absence in the fest, he instantly appears to frighten and shock him out of his wits.