At first the witches do not greet Banquo, and he wonders why they greet Macbeth with such desirable titles and not him. They respond by telling him that "thou shalt get kings, though thou be none." But Banquo is more incredulous about the witches' prophecy than Macbeth, asking if they have "eaten on the insane root/That takes the reason prisoner." He also worries that the prophecy might portend ill for Macbeth, as it comes from such an obviously evil source:
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—
Macbeth, on the other hand, is obviously affected by the prophecy, especially after he discovers that he has indeed been named Thane of Cawdor. Banquo is more wary than Macbeth, warning his friend (correctly) that even though the prophecy has thus far come true, it still might not end well. Macbeth is already thinking about becoming king, Banquo sees how the whole thing could turn out very wrong. He encourages Macbeth to be careful.