In Act I, Scene 4, the audience begins to learn the extent of Macbeth's ambition. He is named Thane of Cawdor, which of course was one of the preditions of the witches, and his response when Duncan names his son Malcolm Prince of Cumberland (the heir to the Scottish throne) is telling:
The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see.
The intense ambition ("my black and deep desires") are juxtaposed with Macbeth's almost obsequious professions of loyalty to the king earlier in the scene:
Are to your throne and state, children and servants,
Which do but what they should, by doing every thing
Safe toward your love and honor.
Clearly, Macbeth plans to keep his ambitions hidden to everyone except his wife. But it is becoming equally obvious that he plans to act on the prophecy of the witches, and when it is announced that the king will visit Macbeth's castle, the audience can infer that something important will happen there. Macbeth's treacherous thoughts are underscored by the fact that Duncan seems to very much admire and value him, declaring him, as the scene closes, "a peerless kinsman."