In Macbeth, Banquo fights valiantly alongside Macbeth. In Act I, they are both witnesses to the witches' prophecies and, apart from predicting Macbeth's future as king, the witches predict that Banquo's own sons will also be kings. Banquo has considered what this means and is suspicious of the witches' intentions whereas Macbeth is plagued by feelings of "vaulting ambition" (I.vii.27) and consumed by thoughts of killing Duncan, the king.
Act II begins with Banquo as he speaks with his son, Fleance. Banquo cannot stop thinking about what the witches have said; his "cursed thoughts" (7) prevent him from sleeping. Macbeth is his friend and he attempts to discuss with Macbeth the impact of the witches on them both but Macbeth suggests that they should speak about it another time. The audience next sees Banquo in scene iii when he learns of Duncan's death. He is most concerned about Lady Macbeth, worrying, as he would about any lady, about her sensitivities (unaware that she is filled with "direst cruelty" (I.v.40)). He is the person who suggests that they must discuss the "treasonous malice" (II.iii.130) with which they are now faced.
It is apparent, therefore, that Banquo is sincere. Shakespeare has revealed a character who is not afraid of the witches, only wary of them, even speaking up on Macbeth's behalf in Act I, scene iii when Macbeth is visibly shaken by the witches. Banquo is also wise and recognizes the potential malice in the witches' intentions. Banquo is humble and noble and definitely honest, telling Macbeth that he has been dreaming of the witches. He is loyal to the king, and, as he says, his "allegiance" is "clear" (I.i.28). Shakespeare leaves the audience in no doubt as to Banquo's honorable qualities. The audience understands why such a good man is a threat to one as ambitious and self-serving as Macbeth.