This is an interesting question. Shakespeare obviously intended to have Banquo and Macbeth be foils. Banquo is calm, patient, intelligent, rational, and conservative, while Macbeth is emotional, rash, ambitious, impulsive, more a man of action than a thinker, and not completely sane. Macbeth is dominated by his wife, while Banquo's wife is never mentioned. He may be a widower. The two men have equal status before Macbeth murders Duncan and becomes king. After that, Banquo treats him with formal courtesy and deference, although he feels certain that Macbeth was responsible for Duncan's death.
Both men believe in the prophecies of the Three Witches they met on the moor, but Banquo is content to wait without trying to take a hand in changing history; whereas Macbeth is only encouraged to go ahead with what he has already been planning. Banquo believes in loyalty and tradition, while Macbeth seems relatively indifferent to such values. A comparison might be made with Hamlet and Laertes, with Banquo resembling the more intelligent and conscientious Hamlet and Macbeth resembling the rash, emotional, impetuous Laertes.
Macbeth himself offers the best comparison between himself and Banquo in Act 3, Scene 1.
Our fears in Banquo
Stick deep; and in his royalty of nature
Reigns that which would be fear'd: 'tis much he dares;
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valour
To act in safety. There is none but he
Whose being I do fear: and, under him,
My Genius is rebuked; as, it is said,
Mark Antony's was by Caesar. He chid the sisters
When first they put the name of king upon me,
And bade them speak to him: then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown,
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding. If 't be so,
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murder'd;
Put rancours in the vessel of my peace
Only for them; and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!
The "Caesar" Macbeth is referring to in this soliloquy is not Julius Caesar but Octavius Caesar, whose relationship with Mark Antony is dealt with at great length in Shakespeare's Antony and Cleopatra. In that play Antony is portrayed as emotional, sensuous, impulsive, reckless and devoted to a manipulative woman, while Octavius is portrayed as wise, thoughtful, patient, cunning, cautious, and coldly realistic. A very good comparison can be made, following Shakespeare's own suggestion, between Banquo and Octavius, on the one hand, and Macbeth and Antony, on the other. Just like Macbeth, Antony recognizes that Octavius is superior to him in many important respects. Like Macbeth, Antony is somewhat afraid of Octavius, although he is too noble to think of trying to have him assassinated. In Act 2, Scene 3 a Soothsayer warns Antony that he should stay away from Octavius, and Antony reflects:
Be it art or hap,
He hath spoken true: the very dice obey him;
And in our sports my better cunning faints
Under his chance: if we draw lots, he speeds;
His cocks do win the battle still of mine,
When it is all to nought; and his quails ever
Beat mine, inhoop'd, at odds. I will to Egypt:
And though I make this marriage for my peace,
I' the east my pleasure lies.
We all know people who make us feel self-conscious and inferior. We are wise to stay away from them.