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:
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.