Why is Banquo's reaction to the witches' prophecies so different from that of Macbeth?
Initially, both Banquo and Macbeth are intrigued when they hear the witches' prophecies for the first time. When Ross and Angus deliver the news that Macbeth has been named the Thane of Cawdor, both Macbeth and Banquo are astonished. However, Banquo reacts quite differently to the possibility that the witches' prophecies are correct. Macbeth is excited about the possibility of someday becoming king and wants to believe the prophecy. In contrast, Banquo is more discerning and says that the agents of evil often tell part of the truth to earn people's trust before eventually destroying them. Banquo tells 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" (Shakespeare, 1.3.125-29).
Macbeth then begins to contemplate killing the king in order to receive the esteemed title. Banquo's cautious reaction to the witches' prophecy reveals his character: Banquo is a loyal, intelligent individual who is not as ambitious as Macbeth. Banquo reacts differently for several reasons. His prophecy is more ambiguous than Macbeth's and is not as personally intriguing. Banquo's personality also allows him to approach the situation rationally and not get caught up in the potentially dangerous prophecies.
For one, Banquo sees the witches for what they are--supernatural beings who play with the lives of humans. He doesn't put too much stock in the messages, or perhaps his nature is not as ambitious as Macbeth's. We never meet Banquo's wife, but it is possible that she is not the blood-sucking woman that Lady Macbeth is, and that her advice was more loving and sound.
For another, Banquo's prophecy was both better and worse than Macbeth's. The witches told him that he is "lesser than Macbeth, and greater...Not so happy, yet much happier...Thou shalt get Kings, though thou be none." So, from this we know that Banquo's nature is happier and greater than Macbeth's. Perhaps not so in tune to ambition and power plays, but more honest, more loyal, more noble. This is also what makes him a target for Macbeth later, since Banquo's children will become kings. It does not set well with Macbeth that his throne will not be secure for generations of Macbeths to rule while Banquo and his son, Fleance, are flitting around.