How would you describe Lady Macbeth's reaction to Macbeth's letter?
In Act I, Scene 5, Shakespeare has Macbeth send his wife a letter in advance rather than having him deliver the gist of the contents in person. This serves several purposes. For one thing it enables Shakespeare to introduce a major character by herself, so that the dramatist can focus on her character and her private thoughts and emotions. She would not be so open and forthcoming if she were meeting with her husband when she first appears in the play, and her husband would detract some of the audience's attention away from her. It also enables Shakespeare to characterize Macbeth through the eyes of the one person who not only knows him best, but knows his secrets. She thinks he is
...too full of the milk of human kindness
and she wants to
...pour my spirits in thine ear
And chastise with the valor of my tongue
All that impedes thee from the golden round
Which fate and metaphysical aid doth seem
To have thee crowned withal.
By "fate" she apparently means the fact that Duncan and his two sons will be staying overnight in her castle. By passing on some of the guilt for Duncan's murder to Lady Macbeth, Shakespeare tries to preserve some modicum of audience sympathy for Macbeth. This is essential since it is supposed to be the "tragedy" of Macbeth, and so the audience should feel some pity for his fate.
The letter saves time. Without it, Macbeth would have to do a lot of explaining when he first greeted his wife. He would be telling her what the audience already knows—that he met the three witches and they told him he would become king, etc. But since Lady Macbeth already knows these things, she can greet him effectively with:
Great Glamis, worthy Cawdor,
Greater than both, by the all-hail hereafter,
Thy letters have transported me beyond
This ignorant present, and I feel now
The future in the instant. (I.5)