How do the roles of Macbeth and Lady Macbeth, during the murder of King Duncan, inform what kind of characters they are?
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.