Answer:
That they know each other one pretty well. there close enough to call the others bluff or can tell when one i off or in in this case has a glimmer of evil plotting in his eye.
Explanation:
When The end of the excerpt comes to a close he can tell the power is getting to his head but before that macbeth is cheering banquo up saying you may not be king but one of your descendants will. I won't have any and try to keep each other on the up.
