The play actually reveals very little about the past relationship of Lady Macbeth and Macbeth. The first scene in which Lady Macbeth appears, which gives the audience a clue about their relationship, is only in Act I scene 5, where Lady Macbeth reads a letter sent to her by her husband about the prophecies he received from the witches. However, this scene is crucial in establishing the nature of their relationship and the relative strength of Lady Macbeth compared to her husband. Although Lady Macbeth is convinced of the truth of these prophecies and she is determined to act upon them, as evidenced by her soliloquy when she gives herself over to evil, it is clear she doubts her husband's ability to act upon them. Note what she says of her husband:
Yet do I fear thy nature.
It is too full o'th' milk of human kindness
To catch the nearest way. Thou wouldst be great,
Art not without ambition, but without
The illness should attend it.
This is a very interesting comment, because, in Lady Macbeth's assessment, her husband does have ambition, but lacks the crucial "illness" or the willingness to do anything in order to achieve that ambition. This first scene is therefore very important in establishing the nature of the relationship between Lady Macbeth and Macbeth. Lady Macbeth recognises that out of the two of them, her husband is the weaker in terms of his inability to commit evil, and she sees her role as goading and encouraging him to commit those crimes necessary to make the prophecies come true. She, lacking any such moral compunctions as her husband possesses, clearly sees herself as the stronger party.