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1.3: Banquo is the first to notice the three weird sisters on the ride back from battle with Macbeth. He wonders at their natures, sensing something is foul by the fact that they seem to inhabit the earth, yet they don't look as things of the earth. When the witches hail Macbeth with his accursed good news, Banquo comments what they say seems nice, and he wonders why Macbeth looks so afraid.
Rather than be afraid himself, he asks the women to look into his future, to say whether it is good or bad. They tell him that he'll bear a line of kings, though he won't be one, and he will at once be greater and lesser than Macbeth, and happier and less happy than Macbeth.
The witches disappear, and Banquo wonders whether he and Macbeth have eaten "the insane root" since they have seen such fantastical things as these women.
When Ross enters announcing that Macbeth is now Thane of Cawdor (just as the witches prophesied), Banquo asks if the Devil can speak true.
While Macbeth is already hatching his nasty plan, Banquo is cautious. He notes that the deepest consequences can come from trifling with evil, which would tell you nice things in order to bring you over to the dark side. Banquo notices Macbeth is distracted, and agrees to speak with him on it later.
1.4: Banquo is greeted by Duncan as Macbeth is, and though he is given no specific honor, he is told that he is close to the King's own heart. Banquo humbly insists that any seed of greatness that the King plants in Banquo is the King's to reap.
1.6: Banquo goes to Inverness (Macbeth's home) with the King and company. Here, he gives a pretty speech about the home of the martin, judging that if that wonderful bird should make its cradle there, the air must be soft and good. (Banquo, it might be said, is not so astute about how to protect one's family and one's self.)
2.1: Banquo and his son Fleance are up late at Macbeth's house. Banquo can't sleep because he is plagued by "cursed thoughts" that he says nature brings to him in sleep. He meets Macbeth walking in the hall, and tells him he dreamt of the weird sisters, which Macbeth brushes off. Cryptically, Macbeth tells Banquo if he will support his cause, it would be an honor to Banquo. Banquo replies that his allegiance is clear (implicitly an allegiance to good and to Duncan) and the two again agree to talk more later.
2.3: Banquo wakes with all the others upon hearing of the King's murder, and is horrified. While Macbeth is busy making long talk, it is Macduff and Banquo who attend to his wife, who has grown faint on hearing about Macbeth's murder of the guards.
3.1: Banquo already suspects Macbeth of some wrongdoing, as the prophecy has come true but in a most awful way. Instead of ruminating on this, Banquo asks whether his part of the prophecy, that he would sire kings, might come true, too. There is no moral tongue wagging here, as Banquo is interrupted by Macbeth and Lady Macbeth, who invite him to dinner after inquiring where he will be at a certain hour of the day. He, innocent of any bad intentions on their part, tells them he will be on a horseback ride with his son Fleance, but will be glad to attend dinner with them later.
3.3: Banquo returns to Forres and is about to attend the big dinner when he is accosted by the murderers Macbeth sent. He says it looks like rain, and the murderers have at him. At his dying breath, he denounces what he knows to be Macbeth's treachery, and bids his fleeing son to avenge his honor.
Note: Banquo's ghost is written into the following banquet scene, and is shown in some productions, while others keep it in the mind's eye of a guilty Macbeth. The ghost does not speak, but gets his haunting on quite effectively anyway.