What purpose does Act II, scene i, and the conversation between Macbeth and Banquo serve? What evidence is there that Banquo is suspicious of Macbeth?
The conversation between Banquo and Macbeth gives the audience further opportunity to understand the friends' now-diverging characters. At first, both Macbeth and Banquo were noble and loyal (consider the captain's description of Macbeth in Act I); now, however, we know that Macbeth is plotting Duncan's murder, and this scene shows us that he is even willing to lie to his best friend. In regard to the Weird Sisters, Macbeth tells Banquo, "I think not of them" (2.1.27-28). Lies: he's been doing little except thinking of them.
Furthermore, when Macbeth asks Banquo to "cleave to [his] consent" when Macbeth asks, a loyalty which he claims will "make honor for [Banquo]," the loyal and forthright Banquo responds, "So I lose [no honor] / In seeking to augment it, but still keep / My bosom franchised and allegiance clear, / I shall be counseled" (2.1.36-39). Such a response accomplishes two things: first, it shows that Banquo is truly honorable, and, second, it shows that Banquo may be developing some suspicions of Macbeth. Why would Banquo assume that Macbeth would ask him to do something that would go against his conscience unless he already suspected that Macbeth were capable of such action himself?
This short scene shows some characterization of both Macbeth and Banquo. Macbeth asks for Banquo's support when the time comes without telling him why which shows the importance of this alliance is to Macbeth. Banquo replies by saying that he will support Banquo with anything as long as his (Banquo's) conscience can remain clear and he won't lose any honor because of his actions. This should show the cautious support of Macbeth given by Banquo, but also that Banquo is beginning to suspect the Macbeth has poor intentions in mind. Both of these men have been thinking about the prophecies quite a bit. Macbeth is inclined to take action, but Banquo would prefer to wait and see what happens.