In Act III,scene iv of Macbeth, Lady Macbeth, once again, comes to her husband's rescue. Macbeth has just seen the ghost of Banquo (sitting in his chair) and being to rant.
Avaunt! and quit my sight! let the earth hide thee!
Thy bones are marrowless, thy blood is cold;
Thou hast no speculation in those eyes
Which thou dost glare with!
Here, lady Macbeth comes to her husband's rescue making excuses for his curious behavior. (This is important because many nobles are gathered to celebrate Macbeth's crowning as king and for him to be acting so strangely would cast doubt upon his ability to be a proper king.)
Lady Macbeth states that Macbeth has had spells, such as the one he is having, for a long time.
The fit is momentary; upon a thought
He will again be well; if much you note him,
You shall offend him and extend his passion;
Feed, and regard him not.
Lady Macbeth has had to come to her husband's rescue before. When Duncan was found murdered, by Macduff, all were surely trying to find out who had murdered him. Lady Macbeth, knowing that she needed to play the part of the horrified lady of the house, faints upon hearing about Duncan's murder. Macbeth had just admitted to murdering Duncan's servants out of rage (when earlier he indirectly stated that Duncan was fine). Lady Macbeth surely knew that Macbeth would be questioned and she decided to faint to save him.