Upon the first meeting with the witches' Banquo seems to be as fascinated by them as is Macbeth. After they tell Macbeth that he is to be "king hereafter," Banquo eagerly wants to know his own fate. Yet Macbeth's and Banquo's responses are quite different. While Macbeth seems "rapt" with the idea that the prophecies might in fact come true, Banquo is much more skeptical. He warns Macbeth:
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.
In other words, Banquo thinks that the witches may seem to be telling truths, but they are only telling part of the truth. He thinks they may be deceiving them as to the true meaning of their words. He calls the witches "instruments of darkness" capable of betraying men with their words.
Macbeth, though, does not heed this warning. His immediate thought is making the third prophesy come true. His faith in the witches seems to be justified. They have correctly identified him as Thane of Glamis; their prediction that he would become Thane of Cawdor has been immediately fulfilled, so it seems reasonable for Macbeth to conclude that he will become king shortly. The question is how will he become king. Will chance crown him king without his having to do anything, or will he need to commit a deed that he can at this point only shudder to think about?
Banquo seems more cautious, more skeptical, and unwilling to act on the witches' prophecies to him. It must be noted, however, that the witches' prophecies for Banquo are much more obscure and more difficult to achieve immediately. But Macbeth seems to believe the witches' completely. He does not doubt their motives or the truth of their words. This faith in the witches continues throughout most of the play and is partially responsible for Macbeth's downfall