Perhaps in keeping with Macbeth's paradoxical observation that "fair is foul, and foul is fair," he and his wife, who are dopplegangers, exchange roles back and forth as they prepare for the assassination of King Duncan. First of all, Macbeth writes to Lady Macbeth of his conquests; however, she worries that despite his brutal slaying of Macdonwald, Macbeth is too full of the "milk of human kindness."  Therefore, she plans to "pour [her] spirits in [his] ear" and "chastise" him with the "valor" of her speech.  Further, she calls upon the spirits to unsex her that she may stop Macbeth from feeling any remorse for the act he is about to commit. She, also, has Macbeth's words only that "King that shalt be" rather than what the witches actually said which was not so definitive: "that shalt be King hereafter." So, she is firmer of purpose than is her husband, who has now has misgivings about slaying his kinsman. In fact, he tells his wife,

We will proceed no further in this business: 
He hath honor'd me of late, and I have bought 
Golden opinions from all sorts of people, 
Which would be worn now in their newest gloss, 
Not cast aside so soon. (1.7.34-38)

Thus, when a reluctant Macbeth tells his wife that they will not continue their plan of assassination, Lady Macbeth accuses him of being cowardly and tells him she will judge his love by his acts,

From this time 
Such I account thy love. Art thou afeard 
To be the same in thine own act and valor 
As thou art in desire?

Then, she seeks to bolster Macbeth's courage, and outlines a plan for the assassination. Impressed with her boldness, Macbeth says she should have only male children to inherit this "undaunted mettle."  He, then, encourages Lady Macbeth to join him in pretending to be happy before their guests.

False face must hide what the false heart doth know. (1.7.93)

Clearly, in this preparation for Duncan's death, Lady Macbeth takes the leading position, acting as the catalyst for the fulfillment of the prophesy of the witches, encouraging Macbeth, who has some misgivings, while Macbeth praises her for her cunning and courage.

How do Macbeth and Lady Macbeth's roles in Duncan's murder reveal their characters?

With Macbeth, some might say that Lady Macbeth is the more malicious of the two characters, forcing Macbeth to commit the murder. But this is an oversimplification and it is a transference of the patriarchal traditional of placing the majority of blame on Eve in the Garden of Eden. Placing all the blame on Eve is often used as an example by feminist theorists to show how this archetypal story unfairly contributes to the misguided idea that strong females must necessarily be flawed or evil.

In Macbeth, Lady Macbeth is just as troubled as Macbeth is about committing the crime. She may come across to the reader as more at ease with the crime but that is a product of a reading tradition where strong female characters are looked upon with suspicion; whereas, strong male characters are heroic. Granted, neither character here is a hero, but the point is that they are equally culpable and they are both consumed with guilt once the crime has been committed.

While Lady Macbeth is waiting for Macbeth to return from killing Duncan, she says, “Had he not resembled/My father as he slept, I had done't” (II.ii.15-16). She considers if she could have killed Duncan herself but in seeing a resemblance to her father in Duncan, she feels that she could not have done it. This is her conscience manipulating her senses to tell her that what they are about to do is wrong.

Macbeth is direct about his own guilt. “I am afraid to think what I have done” (II.ii.64). He knows what he did was wrong. He doesn't even want to think about it. As the play continues on, the more he thinks about it, the worse his mental state becomes.

When you say the “roles” of Macbeth and Lady Macbeth, this refers to the ways that they act. In Act II, Scene 2, we see the duality of these roles. They are capable of committing murder but are plagued by their own consciences and the guilt of having done so. In other words, they are both capable of good and evil. If there were no goodness in them, neither would be so consumed with guilt. Another thing is that there is an internal conflict in Lady Macbeth and Macbeth. So, an actor playing Macbeth must play the role as if he is in conflict with himself. The same is the case for Lady Macbeth.