Contrast the difference between Banquo and Macbeth with relation to the witches' prophecies.

One of Banquo's first responses to the Weird Sisters is incredulity.  He doubts his senses, asking, "Were such things here as we do speak about? / Or have we eaten on the insane root / That takes the reason prisoner?" (1.3.86-88).  He wonders if the sisters were actually there or if he and Macbeth hallucinated them after, perhaps, eating some wicked plant that affected them thus.  Macbeth's response?  "Your children shall be kings," he says to his friend (1.3.89).  Therefore, while Banquo's first instinct is to doubt, Macbeth's first instinct is to believe.  Macbeth seems pretty trusting, given that these women appeared from nowhere, knew the identities of Macbeth and Banquo, and then vanished into thin air; Banquo is much warier and more cautious.

Further, while Banquo makes conversation with the men who come to tell Macbeth about his new title, Macbeth becomes immediately introspective and secretive.  He begins to hope: "If chance will have me king, why, chance may / crown me / Without my stir" (1.3.157-159).  Banquo seems relatively unaffected by the prophecies, other than to wonder at them, but Macbeth takes the Weird Sisters' words as fact.