Banquo's Suspicions and the Crown's Promise
Banquo knows exactly what Macbeth has done to become king. He opens the scene by acknowledging that Macbeth now has "king, Cawdor, Glamis, all" - exactly as the weird sisters promised - but suspects Macbeth "played'st most foully for't." This shows Banquo isn't fooled by appearances.
What's really eating at Banquo is his own ambition. The witches promised his descendants would be kings, and now he's wondering if their prophecies about him might come true too. He's torn between his suspicions about his friend and his hope for his own family's future.
The "fair is foul" theme appears again here - what looks like Macbeth's triumph is actually built on murder and treachery. Banquo represents the moral compass that Macbeth has completely abandoned.
Key Point: Banquo's knowledge of the truth makes him incredibly dangerous to Macbeth's position as king.
