From the beginning of the play, Banquo appears a barve noble and soldier like Macbeth. Yet, with the rising actions, each one takes his own way which differ from the other's one. This raises Banquo to be the kind foil of the villain Macbeth. Shakespeare builds this technique throughout the play, making Banquo's goodness contrast with Macbeth's wickedness.
At the beginning of the play, the two characters are introduced to the audience as similar characters. The audience first hears of them in the wounded captain's account of their courage in the battlefield. The initial impression of them is of brave and capable warriors. Also, Duncan describes them equally as "our captains" who are worthy of his love and regard. Therefore, they are admired by many people, including the Scottish captain, Duncan and Rosse.
However, the contrast appears immediately after they first come out on the stage. When they meet the witches, their reaction towards their prophecies differ from each other. While Macbeth starts to ask them how he will be king, Banquo just asks them to give prophecies for the sake of fan. Even the witches notice the contrast between the two characters. They say that he is "lesser" than Macbeth in the social position as he will not be a king. However, they say that he is "greater" than Macbeth who has the seeds of evil nature inside him, but Banquo has not. Moreover, Banquo's reaction towards their prophecies is more realistic than Macbeth. That is, he asks Macbeth at once about the credibility of the witches and whether they are a part of their imagination as they are exhausted after the battle.
After the first prophecy comes true, their reaction towards the prophecies also differs. Here, Banquo asks only; "What can the devil speaks true" while this raises Macbeth's ambition as he begins to look forward to the other which is the prophecy of being king. Yet, Banquo becomes very suspicious of the witches and warns Macbeth. He thinks that the witches attract them with the half of the truth and refers to them as "The instrument of darkness".
Like Macbeth, Banquo thinks ambitious thoughts, but he does not translate thoughts into action. Surprisingly, it is Macbeth who gets the reward of his loyalty and becomes Thane of Cawdor unlike Banquo. However, Macbeth tries to increase what he gets and be king. In one of his soliloquy, he confesses that his mind which thinks of the murder disturbs his whole being. He is obsessed with the prophecies and suffers from insomnia. Moreover, Banquo mentions to Macbeth that he has dreamt of the "three weird sisters" which shows that his thoughts are innocent. On the other hand, Macbeth claims that he has not thought of them at all, yet he is going to kill the King.
After the crime, Banquo's knowledge of the witches' prophecy makes him a threat to Macbeth's plotting. Macbeth seems distrustful of Banquo and is driven to still more violence before his crown secure. Banquo suspects that Macbeth has killed Duncan. Yet, he seems to be wise and patient, so he does not take any action towards Macbeth. However, the witches' prophecy for Banquo that his descendants will rule Scotland make Banquo an enemy to Macbeth, so he has Banquo murdered.
In conclusion, Banquo's ghost haunts Macbeth at a banquet, and this vision makes the Scottish lords suspicious of their new king. In a sense, Banquo's character stands as a rebuke to Macbeth since he represents the path Macbeth chooses not to take. In this path, ambition does not lead to betrayal and murder. Appropriately, then, it is Banquo's ghost, and not Duncan's, that haunts Macbeth. In addition to embodying Macbeth's guilt for killing Banquo, the ghost also reminds Macbeth that he does not emulate Banquo's reaction to the witches' prophecy.