Banquo remains noble throughout the play, but Macbeth begins the play noble and then becomes corrupted.
Banquo is also a nobleman and a solider. He is a friend of Macbeth’s. He is described as “Thane of Lochaber, a general in the King's army” in the character list (p. 7). He is cautious, and does not like things he cannot understand.
When the witches appear, Banquo is immediately suspicious and interrogates them.
What are these(40)
So wither'd, and so wild in their attire,
That look not like the inhabitants o’ the earth,
And yet are on't? Live you? or are you aught
That man may question? (Act 1, Scene 3, p. 12).
Macbeth is generally a man of few words, but Banquo is very descriptive and asks a lot of questions without waiting for an answer. Macbeth just simply tells them to talk if they can.
Banquo is a very sensitive and insightful man. He watches Macbeth closely, and is disturbed when he seems to not react as expected to the supposedly good news that the witches have predicted his promotion. It is subtle, but it is the first sign that he and Macbeth are beginning to grow apart.
Banquo is concered about listening to the witches, saying, “How can the devil speak true?” (p. 15). He tries to warn Macbeth that no good can come of listening to the witches.
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's(135)
In deepest consequence (Act 1, Scene 3, p. 15).
While Macbeth latches on to the prophecy and seems to think he deserves these promotions, Banquo is not quite so ambitious. He does not concern himself with the prophecy until after Macbeth kills Duncan. At that point he is more concerned with what Macbeth might do.
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't: (Act 3, Scene 1, p. 40)
Banquo worries that Macbeth has done something he should not have. When he tells Macbeth that he had a nightmare about the witches, and Macbeth tells him he hasn’t thought about it, he knows trouble is brewing.
Ultimately, as we know, Macbeth kills Banquo.