It is in Act I, Scene v that Lady Macbeth states clearly her understanding of her husband's political ambitions, as well as his basic nature. She knows Macbeth will hunger for the crown, but she doubts that he will be able to act as necessary in order for him to take the crown for himself. She is afraid that Macbeth is "too full o' th' milk of human kindness" to murder Duncan:
[Macbeth] would be great,
Art not without ambition, but without
The illness should attend it.
By Act III, Scene ii, even though Duncan is dead by Macbeth's hands and Macbeth now rules Scotland, Lady Macbeth still demonstrates a lack of confidence in her husband. She speaks of dwelling "in doubtful joy," and her concern does not seem to focus on Banquo's heirs, as does Macbeth's. She watches her husband closely, questioning why he chooses to be alone instead of distracting their guests so that they won't have time to think of Duncan and his death. Macbeth is alone because he has just met with the murderers he has hired to kill Banquo, but Lady Macbeth assumes he is brooding over Duncan's murder:
Things without all remedy
Should be without regard: what's done is done.
When Macbeth starts talking about Banquo, she advises him to disguise his feelings and "Be bright and jovial among your guests tonight." When he then persists in speaking of Banquo, she tells him to stop: "You must leave this." She also tries to calm him by pointing out that Banquo and his son are not going to live forever. (This is a great example of dramatic irony since Macbeth has just arranged for their murders.) The scene, taken in its entirety, shows that Lady Macbeth feels she must continue to prop up her husband before he drags them down into destruction.