In Macbeth, analyze the way in which Banquo serves as a foil to a central character and thus helps to expand your understanding by contrast. Please use supporting detail and evidence from the text.
This is a great question, and I would argue that Banquo serves as a foil to Macbeth and helps us more clearly see the mistakes that Macbeth makes. After all, Banquo not only is present for the witches' prophesies regarding Macbeth, but he's also part of the prophesies himself. However, while Macbeth chooses to act on the witches' predictions and tries to forcefully make the prophecy come true, Banquo chooses not to act and to simply allow fate to take its own course. As a result, Banquo can be seen as a foil for Macbeth because he takes the opposite course of action and, though he's murdered halfway through the play, one could argue that he enjoys a better fate than Macbeth.
As a reminder, let's take a look at parts of the witches' predictions regarding Macbeth and Banquo in Act 1, Scene 3. Here's the part of the prophesy that concerns Macbeth (all quotes come from eNotes' online version of the text):
First Witch: All hail, Macbeth! hail to thee, thane of Glamis!
Second Witch: All hail, Macbeth, hail to thee, thane of Cawdor!
Third Witch: All hail, Macbeth, thou shalt be king hereafter! (50-52)
Now, here's the part of the prediction that concerns Banquo:
First Witch: Lesser than Macbeth, and greater.
Second Witch: Not so happy, yet much happier.
Third Witch: Thou shalt get kings, though thou be none:
So all hail, Macbeth and Banquo! (68-71)
So, as we can see, there's clear textual evidence that both Macbeth and Banquo play a part in the witches' predictions. However, the way the two men respond to this prediction is completely different. Macbeth decides to take matters into his own hands and forcefully make his part of the prophecy come true. This decision leads him to murder his way to the crown of Scotland. By extension, Macbeth's rash decisions also lead to his unpopularity and subsequent downfall. Banquo, however, takes a very different course of action. Indeed, he more or less sits back and does nothing, allowing his fate to unfurl naturally. Also, though Banquo dies, it's suggested that his line becomes a royal dynasty (which is totally possible, as Banquo's son narrowly escapes murder). Indeed, in Act 4, Scene 1, Macbeth sees a vision of eight kings accompanied by Banquo, and the implication here is that, though Banquo has died, his family will ultimately become a royal dynasty. Thus, though Banquo dies, he arguably has a better fate than Macbeth, who dies as a hated dictator with no family.
As we can see, Banquo is a foil for Macbeth because, rather than trying to force the witches' predictions to come true, he passively allows fate to take its course. Moreover, Banquo's fate seems to be preferable to Macbeth's. As such, Banquo is a foil not only because his actions (or lack thereof) contrast with Macbeth's actions, but also because his fate suggests that Macbeth might have had a happier fate had he not tried to force the witches' prophecy to come true.