Banquo was a great soldier, just like Macbeth. They fought side by side. They were both fierce warriors. They were both ambitious. On the battlefield, their swords steamed from the enemy's blood. In Act One, Scene Two, the sergeant highly praises them in his comments to King Duncan:
So they doubly re-doubled strokes upon the enemy.
Whether they meant to bathe in fresh, bleeding wounds,
Or create another Crucifixion scene,
I cannot tell.
Clearly, Banquo and Macbeth are good friends and good soldiers. We see ambition from Banquo when when the weird sisters give Macbeth his prophecy. In Act one, Scene Three, he comments to the witches that they have said great things about Macbeth, but what about me, he questions:
You greet my noble partner
With current grace and great predictions
Of having nobility and of the hope to be king,
that he seems carried away as well. Only you don’t speak to me.
Clearly, we can see the respect Banquo has for Macbeth. He addresses him as his "noble partner." Clearly, the two are great friends. No doubt, Banquo greatly respects his "noble partner," but he is ambitious and desires a prophecy of his own:
If you can look into the future,
And say what will happen, and what will not,
Then speak to me, who doesn’t beg or is afraid of
Your favors or your hateful spells.
In this, we can see that Banquo is ambitious. Still, all is well between Banquo and Macbeth until Macbeth murders King Duncan. Then Macbeth begins to fear that Banquo will suspect him as the murderer. Indeed, in Act Three, Scene One, Banquo does begin suspecting that Macbeth has murdered King Duncan:
You have it now, King, Cawdor, Glamis, all,
As the weird women promised; and, I’m afraid,
You played most foully for it.
Then Banquo begins to think about his prophecy with hope:
Still it was said
It would not be passed to your children,
Only that myself should be the root and father
Of many kings. If they told the truth,
As their speeches shine on you, Macbeth,
Why, by the truths made good on you,
Might they not be my prophecies as well,
And set up my hopes?
Here we see that Banquo is ambitious even after hearing of Duncan's death.
Now, Macbeth has fear of Banquo's knowledge and with good reason. Banquo states that he will not keep quiet:
But I’ll be quiet; no more.
Banquo is thinking about his own prophecy. He is thinking that if Macbeth were out of the way, his children could become king.
We do know that Banquo is deeply troubled by King Duncan's death. In Act Three, Scene One, when Banquo hears of the murder, he prays that it isn't so:
Too cruel any where.
Dear Duff, I beg you, contradict yourself,
And say it is not so.
In this, we can infer that Banquo cared about King Duncan and would not have ever attempted what Macbeth has done. For this reason, Macbeth is more ambitious than Banquo. Banquo was ambitious but of a lesser degree. He would never have committed murder to ensure his son was king.