When Macbeth receives the new title (Thane of Cawdor) Banquo’s first reaction is that of surprise: “What! Can the Devil speak true?” (I.iii.107). The word “What” followed by the sign of exclamation needs to be noted. It projects Banquo’s mood – in his psyche he has been trying to understand the power of the prophecies. However, he soon recovers himself from this situation and warns Macbeth about the dark side of these prophecies: “And oftentimes, to win us to our harm, / The instruments of Darkness tell us truths; Win us with honest triffles, to betray’s / In deepest consequesnce.” (I.iii.122-26) Banquo is cautious about the evil hidden behind the prophecies and therefore he does not forget to consider the evil effect that may harm him and Macbeth.
While Macbeth receives this news with surprise and joy, Banquo responds with a warning to his friend. Banquo replies that devils often tell half-truths in order to “win us to our harm” (I.iii.121).
Banquo is more sensitive to the fact that the witches represent evil and therefore if their prophecy has come true it must be a trap. He warns Macbeth in this quote to not be lulled into a sense of security and happiness by the prophecy, there will be something that comes along with it that will cause more harm than happiness. Be careful he tells his friend, evil often clothes itself in a shell of outer goodness only to lure you into a vicious trap that consumes you.
Even though Macbeth's prophecy has come true, Banquo does not put any faith in its legitimacy. Unlike Macbeth who is happy, Banquo is worried and fearful of the prophecy coming true.
Banquo responds with suspicion. In Act III, scene i, lines 1-3, he says, "Thou hast it now: king, Cawdor, Glamis, all. As the weird women promised, and I fear thou play'dst most foully for't."
There is a definite rift forming between the former friends, and even Macbeth realizes it which is why he hires three murderers to kill both Banquo and his son, Fleance.
Of course, Banquo takes some comfort in the truth of the prophecy from the witches. Macbeth has been said to be king, and he is. They also, however, said that Macbeth would not have sons on the throne...that honor passes to Banquo. So, Banquo continues with, "It should not stand in thy posterity, but that myself should be the root and father of many kings" (Act III, sc i, lines 4-6).
His reaction is one of suspicion and expectation of honor for his children and grandchildren as future kings of Scotland.