in act 1 scene 3
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I'm thinking you mean this aside:
Glamis, and thane of Cawdor!
The greatest is behind.
Here Macbeth is confessing his secret desires to the audience. Just a little while earlier he was joking with Banquo about the witches' prophecies. Now he is actively thinking and plotting to become king. Macbeth is very easily manipulated and has a deep ambition to become King, even if he has to kill Duncan to do it.