In act one, scene three, the Three Witches meet Macbeth and Banquo on the heath and proceed to give them favorable prophecies. The Three Witches call Macbeth the Thane of Glamis, Thane of Cawdor, and future king of Scotland, which is astonishing and perplexing to Macbeth. Banquo then requests that they offer him a prophecy and the witches tell Banquo that he is "lesser than Macbeth and greater" and that his descendants will be kings, even though he will not be one.
Shortly after the Three Witches disappear, Ross and Angus arrive and inform Macbeth that he has been given the title Thane of Cawdor. Macbeth is surprised and excited by the news and believes that the best is yet to come. Macbeth then asks Banquo if he is thrilled that his descendants will be kings and Banquo responds by saying,
BANQUO: But ’tis strange.
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray ’s
In deepest consequence. (1.3.124-127)
Banquo's response reflects his discerning, cautious nature. Unlike Macbeth, he is less likely to accept the witches' prophecies at face value and does not fully trust them. He entertains the idea that the witches could be "instruments of darkness" and does not overreact to their prophecies. In contrast, Macbeth does not question the witches' intentions and reveals his ambitious nature in an aside by saying,
MACBETH: This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth? I am thane of Cawdor.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? Present fears
Are less than horrible imaginings. (1.3.133-141)
The fact that Macbeth entertains the idea of committing regicide emphasizes his ambitious nature, which is his character flaw. Macbeth also lacks perspective and is significantly less discerning than Banquo. As the play progresses, Macbeth continues to place his faith in the Three Witches and ends up assassinating King Duncan.