In line 52 of Act I scene 3 of Macbeth, Banquo asks Macbeth why he is startled and seems to fear things that sound so fair. Why is this important?

This is an excellent observation. Let us just remind
ourselves of the context of this remark made by Banquo. Clearly, whilst listening to the
prophecies that the witches give him regarding his future rise to power, Macbeth, for
some reason, reacts in fear or shock. Note what Banquo says to him straight after the
witches have finished:

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Good Sir, why do you start, and seem to
fear

Things that sound so
fair?

These lines indicate
that Macbeth has not greeted these prophecies with joy or happiness. I think this is
important for the following two reasons.

Firstly, it
suggests strongly that Macbeth has already contemplated internally making some kind of
bid for the crown, either through murder or through some other method. This therefore
indicates that he is a very ambitious man. What makes him react so strongly to the
prophecies is that they strike a chord with hopes he has already
had.

Secondly, what is key to realise is that it is Banquo
who notices this, and therefore could come to the same conclusions as myself. This could
be used to great effect by a director to make Macbeth initially suspicious of Banquo.
Has Banquo somehow been able to see into Macbeth's heart and see his murderous
intentions and his powerful ambition? The potential is there to exploit this situation
and clearly point towards the fear and suspicions that Macbeth has of Banquo, which of
course leads to Banquo's murder.