In Macbeth, why is Banquo afraid after Duncan's death in Act II?

In Act II, while he is a guest at Macbeth's castle in Inverness, Banquo seems to be afraid of his own thoughts and imaginings. He is worried about what the future holds as he gazes at the starless sky as a potent of dark things to come—"Their candles are all out" (2.1.5).

When Macbeth approaches, Banquo nervously calls for his sword, but is reassured that a friend is there. Macbeth then inquires of Banquo why...

In Act II, while he is a guest at Macbeth's castle in Inverness, Banquo seems to be afraid of his own thoughts and imaginings. He is worried about what the future holds as he gazes at the starless sky as a potent of dark things to come—"Their candles are all out" (2.1.5).

When Macbeth approaches, Banquo nervously calls for his sword, but is reassured that a friend is there. Macbeth then inquires of Banquo why he is "not yet at rest." Banquo replies that he has dreamed of the weird sisters, adding, "To you they have show'd some truth" (2.1.20). This remark implies that they have predicted Macbeth's becoming Thane of Cawdor. However, Macbeth pretends that he has given the witches no thought: "I think not of them." He probably does this in the hope of preventing any suspicion of him by Banquo after he later completes his intended act against the Chain of Being (the murder of King Duncan).

Perhaps, though, Macbeth reconsiders his words, for he may now recall how deeply moved he was at the time of the meeting with the witches, as well as how Banquo observed aloud, "Look, how our partner's rapt" (1.2.143). If he acts disinterested now, Banquo may again become suspicious, wondering why he denies that he was not also disturbed by the predictions they heard. For whatever reason, Macbeth speaks again to Banquo, this time as though wishing to draw Banquo closer to him by his joining in his cause:

If you shall cleave to my consent, when 'tis
It shall make honour for you. (2.1.25-26)

But, because Macbeth's intentions are nebulous, Banquo is suspicious. So, he replies cautiously,

                       ...So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counsell'd. (2.1.27-30)

This rather ambiguous answer is, nevertheless, an implication that Banquo suspects Macbeth of some dishonorable intentions. Hearing these words, Macbeth makes no reply, only bidding Banquo good night: "Good repose the while!" and they both make their exits.