This is a very important scene in the play as it is the first scene in which we meet Macbeth and it is also the scene in which the witches deliver their prophecies to Macbeth that do so much to shape the rest of the play. Key elements include the way in which Macbeth responds differently to the prophecies than does Banquo, and then the way in which Macbeth is so rapt with what has happened that he engages in soliloques discussing his reaction to the prophecies in front of his friends and soldiers.
Any actor playing Macbeth has to focus a lot on Macbeth's response to the prophecies. Note what Banquo says to him after he has heard the glorious future that the witches prophesy for him:
Good Sir, why do you start, and seem to fear
Things that do sound so fair?
Clearly then, while responding to the prophecy of the witches, Macbeth must show evidence of fear on his face, which indicates he had already contemplated making a bid for the crown. This would shape his performance in the scene, as Banquo drawing attention to this might heighten his guilt or the sense of being found out. When Macbeth addresses the witches after they deliver Banquo's prophecy, it is likely that he will be desperate as he calls for them to stay and not leave. His desire to speak with them indicates an agitation of mind that would need to be conveyed.
One of the more interesting elements of this scene is the way in which Macbeth internally debates what he should do about the prophecy. This is a key part of the scene for an actor to focus on, and his performance must indicate the tension within Macbeth as he debates his position with such lines as:
This supernatural soliciting
Cannot be ill; cannot be good...
Being disturbed by Banquo might again produce a somewhat guilty response as Macbeth tries to cover up his contemplation of murder.