In Act 2, Scene 1 of Macbeth, Banquo and his son Fleance are uneasy on the night's watch. Banquo is startled when Macbeth enters, surprised to see he is still up. Banquo tells him the king is asleep and mentions that he had a dream about the witches.
Macbeth claims that he has not given them much thought. Macbeth asks that they talk about it later and they leave. Once alone, Macbeth reflects on his plan and on subjects like sleep, death, and evil deeds.
He contemplates his dagger, first seeing it as a vision, then as a real weapon that he draws from his belt. Is this a dagger which I see before me, the handle toward my hand? Come, let me clutch thee.
Startled by a bell, He moves on to kill Duncan. The conversation between Banquo and Fleance is one of many in the play where characters talk about bad omens and ill feelings of what is to come. In addition, Shakespeare uses dramatic irony in this scene.
Banquo is startled when Macbeth appears, but puts his sword away. Banquo does not know what the audience knows. Macbeth has evolved from an honorable character to a deceitful one. Macbeth is the one they should be guarding against. Once Macbeth is alone and away from his wife, his conscience provokes another moment of hesitation.
He sees a vision of his dagger, and whether it is a supernatural influence or his guilty conscience is unclear, even to Macbeth. This vision foreshadows the madness that will haunt Macbeth after the murder.