Macbeth temporarily deals with his erratic behavior after seeing Banquo's ghost in his place at the banquet by telling his guests that he has a strange disorder which does not even disturb people who know him well. Then, he offers a toast.
In Act III, Scene 4, a banquet has been prepared for the noblemen Ross and Lennox and various lords and their attendants. But, prior to sitting with his guests, Macbeth talks with the murderer whom he has sent to kill Banquo and his son Fleance. When the murderer reports that Banquo is dead, but Fleance has escaped, Macbeth says in an aside "Then comes my fit again."
The Elizabethans believed that ghosts of dead people could come to earth, and when people were troubled, ghosts were especially likely to appear. Such is the case with Macbeth. Since becoming king after his murder of Duncan, he recalls the predictions of the three witches, who have told Banquo that his sons would be kings, and now Macbeth begins to worry that he cannot hold onto the crown. It is in this disturbed state, then, that he imagines that Banquo's ghost sits in his place, and he tells Lady Macbeth privately that he looks upon "that/Which might appall the devil" (3.4.63-64)