There are several pivotal scenes that illuminate the character of Lady Macbeth. The first major scene is Act I, scene v:LADY MACBETH:
Give him tending; He brings great news.
The raven himself is hoarse That croaks the fatal entrance of Duncan(40) Under my battlements. Come, you spirits That tend on mortal thoughts, unsex me here And fill me, from the crown to the toe, top-full Of direst cruelty! Make thick my blood, Stop up the access and passage to remorse,(45) That no compunctious visitings of nature Shake my fell purpose nor keep peace between The effect and it! Come to my woman's breasts, And take my milk for gall, you murdering ministers, Wherever in your sightless substances(50) You wait on nature's mischief! Come, thick night, And pall thee in the dunnest smoke of hell, That my keen knife see not the wound it makes, Nor heaven peep through the blanket of the dark To cry, “Hold, hold!”(55)In this scene, Lady Macbeth is calling on supernatural spirits to give her the power of a man. She asks that these spirts "take [her] milk for gall" implying that the milk in her body is a direct expression of her womanhood, but it needs to be replaced with bravery. Lady Macbeth, at this point, feels no pity for the poor King Duncan, but instead becomes overwhelmed with the idea of becoming a queen.
The next scene that demonstrates how Lady Macbeth steps outside of her "natural" (being that she is a woman, thus supposed to be nurturing) nature is when Macbeth is having second thoughts about murdering Duncan, whom he has respect for. Act I, scene vii:
What beast was't then That made you break this enterprise to me? When you durst do it, then you were a man;(55) And, to be more than what you were, you would Be so much more the man. Nor time nor place Did then adhere, and yet you would make both: They have made themselves, and that their fitness now Does unmake you. I have given suck, and know(60) 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.(65)Again Lady Macbeth is stepping outside of her natural womanly role by explaining that if it were her who made the promise to smash her own baby's head for Macbeth, then she would most certainly keep it. Shakespeare's continuous reference to motherhood is to indicate to the reader how much more like a man Lady Macbeth is at the beginning of the play, compared to her husband.
However, Lady Macbeth is a dynamic character, and the tables turn. In Act V, scene i, Lady Macbeth is caught sleepwalking and washing her hands as she states:
Out, damned spot! Out, I say! One–two— why then ’tis time to do't. Hell is murky. Fie, my lord, fie! A soldier, and afeard? What need we fear who knows it, when none can call our power to account? Yet who would have thought the old man to have had so much blood in(35) him?
Her guilt about the murder of King Duncan has overcome her to the point that she tries to wash the "spot" of blood from her hands in her sleep. The guilt pushes her to the edge of remorse that she ends up committing suicide.
Her character is one that shifts, as Shakespeare shows that we cannot run from our nature.
Another scene during which Lady Macbeth shows the nature of her character is in Act 3 Scene 4 just after Macbeth has seen the ghost of Banquo. Lady Macbeth tries to settle the crowd by telling them that her husband frequently suffers from spells. She turns to Macbeth and asks him if he is a man to which he replies that he is being forced to look upon the Devil. She says,
"O proper stuff! This is the very painting of your fear: This air-drawn dagger, which, you said, led you to King Duncan. O! these flaws and starts (Imposters to true fear), would well become a woman's story at a winter's fire, authoris'd by her grandam. Shame itself! (III.iv.59-65)
Lady Macbeth chides Macbeth's manhood and tells him that he is foolish to allow himself to be so afraid. On earlier occasions, she has also said similar things to Macbeth, so it is obvious that she thinks him weak and unwilling to really go after the things that he wants.
What an excellent answer. One other moment which reveals the complexity of Lady Macbeth's character is the actual murder of Duncan. She has, to some degree, pushed Macbeth into this deed; she hatched the exact plan; and she was supposed to be the executor of that plan. She does get the guards drunk, and she does walk into Duncan's room to kill him. In the end, though, she could not complete the task.
She does have this one moment of weakness in the midst of all her more "manly" talk and actions. It's only a brief moment, and after Macbeth kills Duncan, she goes and stages the crime scene so as to frame the innocent guards.
Because she could not actually commit this murder, her crimes/sins are limited to complicity. This is a telling moment though, and is a foreshadowing that she does have a more "human" nature than we see from her early on in the play. We should not be surprised that the same woman who claims "a little water clears us of this deed" later tries to wash away her guilt but to no avail.