In Act One, scene five, of Shakespeare's play, Macbeth, Lady Macbeth delivers a soliloquy that allows the audience to more clearly understand the kind of woman she is: scheming, ambitious and deadly.
Lady Macbeth's greatest concern is that her husband will not be strong enough to do what needs to be done to realize her ambitions— he must kill Duncan. She says that he is too kind:
Yet do I fear thy nature;
It is too full o’ the milk of human kindness (15)
To catch the nearest way. (I.v.14-16)
She goes on to say that Macbeth has the ambition he needs to be king, but not the evil nature he must have.
As she speaks, seeing her husband in her mind's eye, she feels that if she can talk to him, she may be able to motivate him to commit murder. And so she calls on him to hurry home so that she can pour her poisonous words into his ear.
Hie thee hither,
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue... (lines 22-24)
In Act Three, scene one, Macbeth's soliloquy reflects his concerns of what might stand in his way of remaining king. Being king is nothing if he cannot hold that title safely. He has already forfeited his immortal soul by killing Duncan, and, if Banquo reveals the witches' prediction to the other members of the nobility, the King's murder will have been for nothing and Macbeth will lose all. He knows that Banquo is a brave and loyal man who cannot be corrupted. Banquo is the only one Macbeth fears. It is at this point that Macbeth understands that he must kill Banquo.
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’Tis much he dares, (55)
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valor
To act in safety. There is none but he
Whose being I do fear... (lines 53-59)
There is certainly irony when comparing these two speeches: Lady Macbeth is committed early on to killing Duncan but fears that Macbeth won't have the guts to do it. By Act Three, Macbeth is more accustomed to killing: he arranges for hired assassins to kill his best friend, Banquo. By the end of the story, both characters will have completely exchanged positions as opposed to how they both felt at the beginning of the play.
An example of a thesis statement might be:
At the start of the play, Lady Macbeth is committed to killing Duncan and fears that Macbeth will be unable to follow through; however, by the start of Act Three, Macbeth is now independently making his own arrangements for murder, without Lady Macbeth's knowledge, and he seems completely comfortable now in doing so, even though the target is his best friend, Banquo.