Final answer:
Banquo acknowledges the truth in the witches' prophecies to Macbeth, who, in turn, is showing early signs of internal conflict and guilt as the prophecy begins to unfold, with Lady Macbeth driving his ambition.
Step-by-step explanation:
The exchange between Banquo and Macbeth about the witches and their prophecies is a pivotal element of Shakespeare's play, Macbeth. After the witches vanish, Banquo is skeptical and wonders if they are real or a product of eating the 'insane root.' Macbeth, intrigued by the prophecies, becomes entangled in his ambitions - A phenomenon which Lady Macbeth notices when he struggles to wash off the 'filthy witness' of Duncan's blood from his hands, signifying his guilt and inability to escape the moral ramifications of his actions.
In the quoted dialogue, Banquo references his dream about the witches suggesting that their prophecies have some element of truth, as Macbeth has already been granted the title of Thane of Cawdor, one of their predictions. Macbeth's inability to say 'Amen' indicates his internal conflict and the beginning of his moral decline. Lady Macbeth's acute awareness of their situation and her resolve in the face of Macbeth's growing anxiety highlight her role in spurring his dark ambitions forward.