Significantly, both of these characters are used to show the increasing discontent amongst the Scottish lords of Macbeth's rule and the way that he abuses his power. Interestingly, this occurs through conversations that these two characters have with unnamed others that allow them to fill the audience in on happenings that occur between the central scenes and how the Scottish kingdom is suffering under Macbeth's rule.
Consider, firstly, Ross's conversation with the Old Man in Act II scene 4. The disturbing unnatural events that they relate clearly show divine disapproval of the crime of regicide that has been committed. Although Ross in this scene is not openly suspicious of Macbeth, the way that he discusses the various natural omens that occurred as a result of the regicide clearly indicate that he is suspicious of some dark and fell purpose.
Secondly, consider the conversation that Lennox has with another Lord in Act III scene 6. This is an openly sarcastic speech, where Lennox clearly suggests that Macbeth was behind Banquo's murder and the murder of Duncan. Note the following example of sarcasm:
The gracious Duncan
Was pitied of Macbeth--marry, he was dead--
And the right-valiant Banquo walk'd too late;
Whom, yo may say (if't please you) Fleance kill'd,
For Fleance fled. Men must not walk too late.
The sarcastic way in which Lennox draws attention to the claim that Fleance killed his father, because he fled, clearly suggests the way that Macbeth's fictions are beginning to be penetrated and that the Scottish lords are increasingly unhappy with his lies and tyranny.