In act 4, scene 3, of Macbeth, in which Macbeth hosts a banquet, there is actually a lack of humor or comic relief. Instead, the tension grows increasingly high as Macbeth sees the ghost of Banquo, essentially throwing a fit and raising concern among his guests, who recognize that "His highness is not well." Meanwhile, an embarrassed Lady Macbeth attempts to persuade both the guests to stay and Macbeth to get his act together. The scene has a rather chaotic feel to the reader or viewer.
As Macbeth tries to navigate his guilt while maintaining a calm image, and Banquo continues to appear to only him and no one else in the room, Macbeth has trouble remaining relaxed or seeming fine. He continues to talk to the ghost, whom no one else can see, and ultimately the tension comes to a head when Lady Macbeth asks everyone to leave, admitting that Macbeth is not well.
The end of this scene leaves the reader with a tone of defeat, as guilt has begun to consume Macbeth and take away his sanity as punishment for his unacceptable acts of cruelty.
In Act 2, sc. 3, Shakespeare breaks up the tension of the play with the comical scene of the exchange between the Porter and Macduff when Macduff arrives at Macbeth's castle to get Duncan. This is typical move on Shakespeare's part. During the banquet scene in Act 3, sc. 4, there is no humor. When Macbeth sees the ghost of Banquo sitting in his seat and no one else sees the ghost, it is not humorous, it is showing us Macbeth's current mental state. The scene shows how much guilt Macbeth feels over having had Banquo, his good friend, killed. Shakespeare isn't trying to create humor when he has Lady Macbeth make excuses for Macbeth's behavior; he is showing how Lady Macbeth has to cover for her husband. These actions show us the level of desperation of both Macbeth and Lady Macbeth.