When Macbeth brings further news that Duncan is actually coming to spend that night with them, it becomes clear that her role is to seize the moment and facilitate her husband's rise to kingship.
Before the murder
Well, Lady Macbeth uses a number of methods to persuade Macbeth to change his mind. Firstly, she says he has already promised to do it which he infact didn’t even though he did think about it.
What beast was't then
That made you break your enterprise to me? (I, vii)
This is her opening line - simply pointing out that he raised the idea first. She taunts Macbeth's masculinity - calling him a coward:
Art thou afeared
to be the same in thine own act and valour
as thou art in desire? (I, vii)
This is an important part of her approach. Macbeth's rank and fame depend on his courage and bravery. She says he cannot love her:
From this time
such I account thy love. (I, vii)
This personal taunt really hits home for Macbeth. It is unexpected because their relationship is so intense. These three points lead on to the following question - why does Lady Macbeth say she would kill their child?
Lady Macbeth has lost a child when it was very young. It's really shocking when she says she would have smashed it to the floor rather than go back on a promise. This would be the ultimate sacrifice. She makes the point that she knew the joy of being a mother, and would have given that up for Macbeth to be king. She uses terrible, violent imagery as a shock tactic:
I have given suck, and know
How tender 'tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this. (I, vii)
She realises that Macbeth's doubt needs to be overcome quickly and this needs extreme measures. If they delay one night, the chance is gone.
The morning after the murder
In Act 2, scene 3, without any warning, Lady Macbeth faints. Why? This has been argued about ever since the play was first performed. Does she faint to distract attention, because the others might see through Macbeth's elaborate excuses? Or is it because she is genuinely shocked and overcome, and her strength suddenly leaves her?
Well, you could say she's distracting attention, depending on how you read the scene. Certainly her line "Help me hence, ho!" could be said in a theatrical way to distract attention.
But she also seems suddenly to feel alone and scared by Macbeth's words and actions:
Who can be wise, amaz'd, temperate and furious,
Loyal and neutral, in a moment? No man:
The expedition of my violent love
Outran the pauser, reason. Here lay Duncan,
His silver skin lac'd with his golden blood;
And his gash'd stabs look'd like a breach in nature
For ruin's wasteful entrance: there, the murderers,
Steep'd in the colours of their trade, their daggers
Unmannerly breech'd with gore: who could refrain,
That had a heart to love, and in that heart
Courage to make's love known? (II, iii)
Initially, Lady Macbeth had to use all her influence to persuade Macbeth to murder. Now he commits it without consulting her. It is becoming frighteningly easy for him. Also, Macbeth may have been directing some of his angry words at her. His fury and menace would really be frightening, especially as earlier in the play she thinks he would be too mild to kill the king in the first place.
Lady Macbeth is shocked by the guards' murder. She was not prepared for more death:
My hands are of your colour, but I shame
To wear a heart so white...
A little water clears us of this deed;
How easy it is, then! (II, ii)
She thought that the killing of Duncan would be the end of the story.
