Shakespeare did not wish to show the actual murder of King Duncan because, for one thing, it would have been hard to enact on the stage. Instead, in Act 2, Scene 3 he makes a big spectacle of the aftermath of the murder, beginning with the arrival of the drunken porter to open the gate for Macduff, who has been knocking for a long time. The knocking forces Macbeth to come out of hiding. He has to find out why nobody is responding to the racket, and he realizes he can’t pretend to be sleeping through all that insistent pounding at his gate. Very reluctantly he leads Macduff and Lennox to the door of the King’s chamber. When Macduff discovers the bloody body he starts shouting and ringing the alarm bell, summoning everyone onto the stage in their nightgowns. It is a ghostly spectacle with all these people looking like ghosts in their white gowns and moving about noiselessly because they are all in their bare feet.
Macduff suggests to the audience that all the people he summoned will look like ghosts when he cries out
As from your graves rise up and walk like sprites
To countenance this horror.
Since the bloody scene inside the King’s chamber cannot be shown, it is described by both Macduff and Macbeth. Shakespeare always relied more on dialogue than on action because action is difficult to display effectively on a small stage, and furthermore the appeal of his plays is always in the spoken words.
Lady Macbeth does not actually faint but pretends to be about to faint because she is supposedly overcome with emotions, both from learning that Duncan has been murdered and because of the descriptions of the scene inside the King’s bedchamber given by Macduff and her husband. This is pure fakery. This tough lady has already been inside the bedchamber to return the bloody daggers and to smear the faces of the two grooms with Duncan’s blood. She pretends to faint because, for one thing, she believes it would be appropriate for her to faint under the existing circumstances, but also because she would like to break up this big gathering before too many awkward questions have been asked. She didn’t expect her husband to have to be present when the King’s body was discovered. Her act takes some of the pressure off Macbeth, who is not doing a very good job of acting surprised and aggrieved, and not doing a particularly convincing job of explaining why he rushed into the King’s bedchamber and murdered the two attendants before they had a chance to tell their stories.
Her pretended fainting spell has the intended effect of breaking up the gathering. It will give her time to meet with her husband in private and discuss the situation. No doubt it will be Lady Macbeth who will do most of the talking, because she has to keep giving her husband badly needed advice and encouragement.
Lady Macbeth may have gotten the idea of pretending to faint from what Macduff tells her when she first appears in her nightgown and asks
What’s the business
That such a hideous trumpet calls to parley
The sleepers of the house? Speak, speak!
To which Macduff replies
O gentle lady,
‘Tis not for you to hear what I can speak.
The repetition in a woman’s ear
Would murder as it fell.
Lady Macbeth does not faint because of her husbands description of Duncan's murder scene, she faints as a diversion. The murder plan was clear, kill King Ducan and frame the Chamberlains. When Macbeth was able to and mistakenly tells Macduff and the other Thanes about the scene, which he should not have know about at all, he directly ties himself and his wife to the murder.
So being a smart gal, Lady Macbeth faints to distract Macduff from asking anymore questions that might implicate her or her husband in teh murder of King Duncan and it works...distraction. Every man wants to help a damsel in distress.
She really faints because macbeth ws about to reveal his involvement in the murder so lady macbeth had to do something to help macbeth