Act two scene three
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Macduff is the unfortunate guy who has to see the dead king first. It is Macduff that raises the sleepers of the castle,
"Awake, awake! Ring the alarum bell. Murder and treason!"
Banquo is just shocked and confused. One wonders if he has Macbeth pegged as the murderer right away but he doesn't voice his suspicions until act 3 scene one.
Macduff is assertive towards Macbeth when he asks why he killed the guards, perhaps the only witnesses to the murder. I never felt that Macduff really bought Macbeth's pretty lame explanation. Really it's Macduff that drives much of the plot forward from that point on.
basically, I think Macduff plays the role of being very suspicious as the fact that Macbeth had killed the chamberlains although Macbeth gives excuses talking about his love for Duncan.
banquo on the other hand is also suspicious but he doesn't say anything yet.