By all accounts, Banquo is a good friend of Macbeth in the beginning. When they first hear the prophecies, Banquo doesn’t take them seriously and Macbeth does not seem to either. They laugh and joke about them.
First, Banquo suspects the witches from the start.
Good sir, why do you start, and seem to fear
Things that do sound so fair? I’ the name of truth,(55)
Are ye fantastical or that indeed
Which outwardly ye show? (Act 1, Scene 3, enotes etext pdf. p. 13)
The conversation is hardly serious, as the two men joke about the prophecies.
Were such things here as we do speak about?
Or have we eaten on the insane root
That takes the reason prisoner?
Your children shall be kings.
You shall be King.(90)
And Thane of Cawdor too. Went it not so?
To the selfsame tune and words. (p. 14)
Later, Banquo begins to feel that the witches are not harmless fun, and might be dangerous. They give him nightmares the night Macbeth murders Duncan. He realizes they seemed to be right about Macbeth.
I dreamt last night of the three weird sisters:
To you they have show'd some truth.(25) (p. 27)
Macbeth lies, saying he has not thought of them.
Let us not forget that there was a prophecy about Banquo to, and as Banquo begins to suspect Macbeth of Duncan’s death, he starts to wonder about his own prophecy that his sons will be king, though he will not. Surely Macbeth will see both Banquo and his son Fleance as a threat.
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't: yet it was said
It should not stand in thy posterity,
But that myself should be the root and father(5)
Of many kings. If there come truth from them—
As upon thee, Macbeth, their speeches shine—
Why, by the verities on thee made good,
May they not be my oracles as well
And set me up in hope? But hush, no more.(10) (Act 3, Scene 1, p. 40)
It turns out Banquo is right. It is not long after this that Macbeth has him murdered. His son only barely escapes.
Please see the first link for the PDF from which I used the page numbers.