To what extent do the circumstances around the killing of Duncan reveal an exchange of the usual gender roles between Macbeth and his wife?
Once the plan to kill Duncan has been set, Macbeth and Lady Macbeth undergo a role-reversal, in that, Lady Macbeth takes shows all the courage. She becomes Macbeth's backbone, nearly forcing him into committing murder.
In Act I, scene 5, Lady Macbeth and Macbeth speak briefly regarding their plan to kill Duncan. Macbeth is extremely nervous and frightened. Lady Macbeth tells her husband that she can read him like a book, and he should be careful. She tells him he needs to deceive the world and play the part of a confident, innocent man (lines 60-64).
Later, in Act I, Scene 7, Lady Macbeth questions Macbeth's desires and motives to wear the crown. She also questions his manhood, claiming that he may be too innocent and pure to commit such an act, even though he longs for the crown (lines 48-59).
*All line numbers are taken from The Norton Shakespeare, based on the Oxford edition.
This can be seen as early as Act 1, sc.5 when Lady M. asks the spirits to "unsex" her because she thinks her husband is too weak to commit the murder. She wants to rid herself of female kindness/weakness in order to help her husband in this deed. Later in scene 7 of Act 1, she tells him he is a coward and not a real man if he doesn't do this deed. Again, taking the role of the aggressive spouse. By act 2, sc.2 we see her taking the bloody daggers back to plant them on the guards and basically telling her husband to "get it together." Through the entire situation she has more of a take charge attitude. She seems to be taking care of the "problem" and her husbands, where usually we think of the husband as the spouse who "takes care of things."