Lady Macbeth's reaction to Macbeth's seeing Banquo's ghost is to immediately cover up her husband's hallucinations. Lady Macbeth recalls the "air-drawn dagger" that Macbeth claimed to see when he killed King Duncan. She knew about that murder obviously, but of Banquo's murder she is ignorant. However, she connects the two hallucinations and suspects that her husband is involved in Banquo's disappearance and murder. She tells Macbeth that the ghost is just "the painting of [his] fear" and that he needs to calm down before anyone suspects any wrongdoing. When he cannot, she takes over and tells all the dinner guests that Macbeth has been ill since he was a small child. She then tells everyone that the party must end because Macbeth needs rest, and she ushers everyone out while Macbeth continues in his hallucinations.
In Act 3, Scene 4, Macbeth is tortured by the appearance of the ghost of Banquo, and Lady Macbeth attempts to preserve order and harmony by reassuring guests that Macbeth's fit is fleeting:
Sit, worthy friends: my lord is often thus,
And hath been from his youth: pray you, keep seat;
The fit is momentary...
Her primary intention is to pacify the potentially harmful effects brought on by Macbeth's outburst and encourage Macbeth to let go of the vision he struggles with. She reminds him that this vision is similar to the one which involved "a dagger of the mind," and she convinces him that this is all a product of his imagination. She urges him to bury his fear and demonstrate his sanity for the sake of the guests:
This is the very painting of your fear:
This is the air-drawn dagger which, you said,
Led you to Duncan.
After this, Macbeth seems to have come to his senses, but only briefly. The ghost reappears, and Macbeth's descent into irrationality becomes clear again. This time, Lady Macbeth is unable to do anything to mend the situation. Therefore, she is forced to tell the guests to leave, stating that Macbeth is not well:
He grows worse and worse;
Question enrages him. At once, good night:
Stand not upon the order of your going,
But go at once.