Act 2, scene 1, is more about Banquo and Macbeth than it is about Banquo and Fleance; although Fleance is there, he plays a very small role, but what should be noticed is that they are about to retire for the night, when suddenly Macbeth pays them a visit. The scene plays three important roles.
First, we are introduced to Fleance, Banquo's son. This has bearing only in that the witches tell Banquo that he will not become king, but that he will have sons who will become kings.
Secondly, the scene reveals just how much Banquo trusts no one, for when he hears the footsteps of Macbeth approaching his chamber, he orders Fleance to give him his sword.This is somewhat peculiar when you consider that he is in the confines of Macbeth's castle, a place where he should feel a sense of security.
The third reason for Shakespeare beginning this Act with Banquo is for Macbeth to determine if he can trust Banquo after he murders King Duncan. Macbeth tells Banquo that he would like to speak to him more about what the witches told them earlier that day when they have time, andBanquo tells Macbeth that he is ready to speak with him whenever he would like. Then Macbeth tells Banquo that
If you shall cleave to my consent, when 'tis,
It shall make honor for you.
Banquo's responses is
So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear
I shall be counseled.
Macbeth has just got his answer; he cannot trust Banquo, for Banquo tells Macbeth that he will listen to Macbeth when the time comes only if what he wants from him doesn't compromise his honor and his allegiance to the King.