Macbeth acts with genuine surprise at the witches' prophecies. He is in disbelief at first. He accepts the prophecies as truth after Ross and Angus ride over to share the good news that he is Baron of Cawdor. Then Macbeth begins thinking to himself about being king. Automatically, Macbeth begins to think of how he could become king. Macbeth thinks of murdering King Duncan and this frightens him:
[Aside.] Those creatures told two truths
As happy prologues to my ascending
The throne. I thank you, gentlemen.
[Aside.] This supernatural meeting
Can’t be bad, only it can’t be good either. If it’s bad,
Why has it given me promise of success,
That began with a truth? I am Baron of Cawdor.
If it’s good, why do I give in to that suggestion
Whose horrid image makes my hair stand on end,
And makes my heart pound so hard they knock at my ribs,
Against my will to stay calm? My current fears
Are less than horrible imaginings.
My thought, whose murder is still only a fantastic idea,
So shakes my manhood, that functioning like a man
Is smothered in unfounded allegations; and nothing is
Only what is not. (Act I, scene iii)
While Macbeth is frightened when he thinks about the price he will pay to have King Duncan's kingship, Banquo seems skeptical in that his sons will become kings. Banquo is not concerned with how it will happen. He seems somewhat skeptical of the witches' prophecies. Macbeth is contemplating murder. He knows that King Duncan will have to die in order for Macbeth to become king.
No doubt, Banquo and Macbeth are looking at the prophecies quite differently:
Banquo is skeptical of the Witches, but Macbeth, driven by a desire for power, considers killing Duncan to gain the crown. Macbeth is overwhelmed by the image, yet his desire for power is still present, as stated in a letter he sends to Lady Macbeth.