Act 3, scene 4 adds to the atmosphere of the play by using the supernatural because it starts to show the doubt in Macbeth’s mind that he will remain king and that his plans will remain undiscovered. He says at the end of the scene (lines 122-123 and 125-126):
“It will have blood, they say, blood will have blood.
Stones have been known to move, and trees to speak.”
“ brought forth
The secret’st man of blood.”
Shakespeare is also showing that, although Macbeth has started to worry that he will be found out, he also has a nature that is starting to become bloodthirsty. “We are yet but young in deed” implies that there is more murder to be done; for suspense and prophetic irony. That is the final line of the scene, as it could leave the audience wondering if there will be more ‘deeds’ to be done.
Shakespeare shows that the overall atmosphere of the scene is trying to be dynamic, but has a hidden gloomy interior, a parallel to how Macbeth is feeling in that scene.
The atmosphere of the scene is described too by Shakespeare in what Lady Macbeth says to her husband when he is frightened upon sighting the ghost.
“This is the very painting of your fear:
This is the air-drawn dagger which, you said,
Led you to Duncan. ”
He is starting to fear being discovered as the murderer, and Lady Macbeth is suggesting that his “heart so white” is creating the image of the ghost, just as his own mind created the dagger that pointed him towards killing King Duncan. A modern audience would see that, once again, his subconscious is showing his innermost thoughts, and trying to influence his next decisions. Shakespeare may be showing the witches hold over Macbeth’s mind at this point too.
Another way that Shakespeare is trying to create suspense and a dark atmosphere in this scene is with his choice of words and the images they create; when the ghost leaves the banquet, Lady Macbeth tells to her husband that he has “displaced the mirth”, as Macbeth thinks about the image he has seen. He calls it a “horrible shadow”, and later on in the same scene he uses the image of “a summer’s cloud”; a dark thing overshadowing the good things in life. Macbeth is king, but his mind is filled with despair and worry. In one of the previous scenes, Macbeth confesses to his wife that he is losing sleep over the deeds he has done; Shakespeare links to the scene with Banquo’s ghost to emphasise how uncertain Macbeth is in his mind.
Also, Banquo’s ghost takes Macbeth’s seat at the head of the table. Shakespeare uses prophetic irony: Banquo may be dead, but his descendents will be kings after Macbeth (as further highlighted in Act 4, Scene 1 with the aspiration of Banquo and many other ghostly Kings who share the image of Banquo.)
Both of these scenes show the audience the changing relationship between Macbeth and Banquo. In Act 1, scene 3, Shakespeare shows the relationship as it was: friendly and supporting one another; and Act 3, Scene 4 shows the relationship how it is for the rest of the play: tormenting each other in different ways; Banquo causing Macbeth to ponder the sanity of his own mind, and Macbeth killing Banquo in order to better himself.
I think that Shakespeare’s use of the supernatural is a clever way to link the happenings of the play (from the witches’ spell, and the death of Macdonwald, Banquo and Macbeth’s meeting with the witches, all the way to the fulfilling of the prophecies and Macbeth’s death). The witches certainly influenced Macbeth's mind and actions, even when he ignored the desires deep inside him (Act 1, scene 4, lines 50-52), he was manipulated by them and by his wife, into becoming a murderer. Shakespeare makes me wonder whether, if Macbeth and Banquo had never stumbled upon the witches, would King Duncan have died? Would the witches have tried to ensnare someone else, or was it their plan for only Macbeth to ‘do their bidding’? In the end, they gained the most: amusement, and evil spreading into the hearts’ of several people (Macbeth and his wife), which seems to be a thing that they feed off.