In Act Two, scene three, of Shakespeare's Macbeth, Macduff arrives early at Inverness to collect the King and go on their way. When they go to waken Duncan, his murder is discovered, along with two bloodied guards who have no idea what is going on, as their drinks had been drugged the night before.
Macbeth murders the guards to keep them quiet, excusing his actions as those of one overcome by the death of his dear King—in that moment, he went crazy.
While all of this is going on, Lady Macbeth pretends to faint. There is no question that she is trying to divert attention away from her husband. First, when they are inside, covered with blood and hear the knocking on the castle gate, it is Lady Macbeth who has the presence of mind to tell Macbeth to clean up and get into his bed clothes, as they should be asleep. She tells him to pull himself together. Certainly, she must be worried about how her husband will react when the murder is discovered; so fainting also gives Macbeth a little more time to pull herself together. This is ironic because fainting would be sometime a squeamish woman would experience: however, Macbeth is the squeamish one, and Lady Macbeth has complete control. She is guaranteeing that everything goes smoothly by calling attention to herself and her "vapors." She is much more sneaky and manipulative—even murderous—than one might expect.