Macbeth and Banquo are both soldiers and noblemen. However, Macbeth is ambitious while Banquo is more reflective.
When the story begins, there is little difference between Macbeth and Banquo except that Macbeth has recently had great victory in battle. They are both strong and brave, and they seem to be friends. When the witches give them prophecies, they seem to imply that Baquo is both lesser than Macbeth and greater, because he won’t be king and Macbeth will, but his sons will be king and Macbeth’s won’t.
Their reaction to the witches is very different. Banquo is immediately confused, suspicious, and somewhat amused. He describes them as looking “not like the inhabitants” of Earth, yet on it.
Live you? …. You should be women,
And yet your beards forbid me to interpret
That you are so. (Act 1, Scene 3, p. 12)
Macbeth wants to know more immediately. When he hears the prophecies, he wants to know how they can be true. He jumps to the logical questions behind what they are saying, rather than focusing on what they look like as Banquo does. They are telling him what he wants to hear, so he is willing to overlook who and what they are.
Stay, you imperfect speakers, tell me more.
By Sinel's death I know I am Thane of Glamis;
But how of Cawdor? (Act 1, Scene 3, p. 12)
When they leave, Macbeth is wishing they had stayed while Banquo is wondering if they have bone gone crazy, and if they were really there.
Were such things here as we do speak about?
Or have we eaten on the insane root
That takes the reason prisoner? (Act 1, Scene 3, p. 12)
While Macbeth focuses on what the witches said they would give him, Banquo focuses on Macbeth’s reaction. He likely knows Macbeth so well that he realizes something is not right. His friend is too interested in these witches, and taking the prophecies too seriously. From then on, Banquo is suspicious of Macbeth.
When Duncan honors Macbeth, he instantly wants to know why he wasn't named the next king. He is shocked when Macbeth is promoted, saying: What, can the devil speak true? (Act 1, Scene 3, p. 15) and he tries to warn Macbeth.
But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's... (Act 1, Scene 3, p. 15)
Macbeth will have none of it though. He is already saying to himself that he is supposed to be king, not Malcolm.
This is the point where Banquo and Macbeth begin to grow apart. Next, Banquo seems well aware that Macbeth has murdered Duncan.
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)
This is also so point where Macbeth begins to worry about Banquo, and decides he is too much of a threat to let live.
Macbeth and Banquo are 'friends' but obviously Banquo cherishes his friendship far more than Macbeth. Banquo is genuinely pleased for Macbeth when the witches tell him that he will be Thane of Cawdor and king and is concerned that Macbeth does not seem so pleased:
..Things that do sound so fair?..
He is equally concerned for Macbeth when the first prophesy is confirmed and Macbeth is then encouraged and begins to believe that he could be king. He warns Macbeth about the witches as he does not have the same confidence in them as Macbeth does:
...tell us truths
Win us with honest trifles, to betray 's
In deepest consequence.
Banquo's trust does begin to waver. He does begin to question Macbeth's reasoning and does feel that, if there is merit in the witches promises then his own sons will be kings. he does not view it however like Macbeth does - that it must be at the expense of all else!
Macbeth, on the other hand, will stop at nothing in the pursuit of his 'destiny' and even though he is haunted by Banquo's ghost, after he has him killed, it is more because of his determination to fulfill the prophesy and because he knows Banquo's son escped than out of any remorse for killing his friend.
Banquo is noble and honoroble but Macbeth only appears noble. His valour that we see at the beginning of Macbeth does not fool us for long, especially in view of the witches prophesies.
Refer to the eNotes study guide and navigate to the summaries and character analyses for an understanding of the realtionships in Macbeth.