After Macbeth becomes king of Scotland, Banquo very wisely treats him with extreme formal courtesy and refrains from making any allusions to the Weird Sisters or to any other circumstances leading to Macbeth's acquiring the title of Thane of Cawdor or acquiring the Scottish crown. Banquo, however, expresses his real thoughts and feelings about these circumstances in a short soliloquy at the very beginning of Act III. In this speech, he says he believes Macbeth has acted foully and that he begins to hope the prophecy given him might be equally true and make him the father of a kingly line.
Thou hast it now--King, Cawdor, Glamis, all
As the Weird Women promised, and I fear
Thou played'st most foully for 't. Yet is was said
It should not stand in thy posterity,
But that myself should be the root and father
Of many kings. If there come truth from them
(As upon thee, Macbeth, their speeches shine)
Why, by the verities on thee made good,
May they not be my oracles as well,
And set me up in hope? But hush, no more.
Banquo obviously has a bad opinion of his former friend, but he does not appear to be as outraged by his strong suspicions as others, such as Macduff, might be. This is apparently because Banquo cannot help seeing that Macbeth's probable murder of King Duncan can be paving the way for the fulfillment of the Weird Women's prophecy that his own children rather than Macbeth's will be the future kings of Scotland. Banquo's behavior toward Macbeth shows that he is being characteristically cautious, patient, and intelligent.
When Macbeth enters the scene, Banquo treats him as the legitimate monarch. It is interesting to observe his respectful behavior after learning from his own lips that he considers Macbeth a murderer, a villain, and a usurper. Banquo consistently addresses Macbeth as "my lord," and allows Macbeth to lead the conversation. This is considered proper behavior in an interview with a king or queen. (In Boswell's famous biography of Dr. Johnson he describes how Johnson once was honored with a private meeting with the King of England. Johnson told Boswell that he did not worry about what he was going to say to the King because a subject should simply respond to questions and not initiate any topics.) Macbeth is already planning to have Banquo and Fleance ambushed at someplace away from the palace, and the conversation, consisting of questions and respectful replies, allows Macbeth to ask such seemingly casual questions as:
Ride you this afternoon?
Is 't far you ride?
Goes Fleance with you?
Still, Banquo does not suspect that Macbeth might already be plotting against him and his son. He is taken by surprise in the third scene of Act III when the three murderers attack in the dark.