In Macbeth, Act II, Scene iii, Lady Macbeth opens the scene. Shortly afterward, Macbeth enters. The conversation between them is about the fact that Macbeth has just recently murdered Duncan and he feels something has changed within him. Macbeth admits that because of the murder he will no longer be able to sleep:
Macbeth shall sleep no more!
Macbeth swears that he heard this pass from the lips of Duncan's guards. Lady Macbeth, angered at the fact that Macbeth seems to becoming weak, "You do unbend your noble strength," decides that she must do something to insure the finger is pointed at someone other than Macbeth. Lady Macbeth exists so as to insure that McDuff and Lennox do not blame Macbeth.
When Lady Macbeth reenters, she tells Macbeth that she has returned the daggers to the sides of the guards and smeared them with Duncan's blood (basically framing them for the murder of Duncan). Lady Macbeth tells Macbeth that she is now just as guilty as him, but she "wears a heart of white." Lady Macbeth states that a little water alone is able to clear them of their deed.
What the scene tells readers about Lady Macbeth is that she sees her husband as weak and acknowledges the fact that she must take charge to insure that Macbeth gains the crown as stated by the witches' prophecy. Lady Macbeth, here, seems to be the far stronger character while, at the same time, her brutal character is uncovered.