Discuss the relationship between Macbeth and Banquo.

Macbeth and Banquo have a very dynamic relationship. In the beginning of the tragedy, Macbeth and Banquo are old friends bonded through their shared time serving as generals and fighting alongside each other for their king, Duncan. Both of the men are also the only ones to see the three witches and receive a prophecy. Macbeth's is as follows, "All hail, Macbeth! Hail to thee, thane of Glamis!/All hail Macbeth, hail to thee, thane of Cawdor!/All hail, Macbeth, thou shalt be king hereafter!" Banquo's is such: "Lesser than Macbeth, and greater./Not so happy, yet much happier./Thou shalt get kings, though thou be none." This is a shared experience that, in theory, should bring the men closer together. However, as Macbeth begins his descent into madness and obsession with the witches' predictions, their relationship takes a turn.
After killing Duncan in the pursuit for kingship and gaining the crown, Macbeth realizes that not only is Banquo is the only other person who heard the witches' predictions, but that their prediction for Banquo states that he shall not be king, but his sons will be. This is a major kink in Macbeth's plans. Macbeth is concerned that Banquo and his son, Fleance, will conspire together in order to take the throne from Macbeth. In his panic, Macbeth does the only thing he thinks will solve this problem-- kill his now ex-best friend and scare away his son. With Banquo murdered, Macbeth loses his only true friend, as this puts a pretty large damper on Macbeth and Banquo's relationship.
In his guilt over both murders he has now been a part of, at a great dinner with all of the other nobleman, Macbeth sees Banquo's ghost. Even in death, Banquo is serving as a reminder of the wrong doings Macbeth has committed in his greed and ambitious behaviors. Even after death, the two men's relationship continues to change. Now, rather than a friend or a direct threat, Banquo serves as Macbeth's conscious. He is present to remind Macbeth that what he is doing is a sin and he will be punished if he continues down this path.

Quote Source:
http://www.shakespeare-online.com/plays/macbeth_1_3.html

Initially, the relationship between the two men is perfectly amicable. They are friends and fellow nobleman, loyal servants of their king, Duncan. Yet as the play progresses, things turn sour, with each man gradually perceiving the other as a threat, albeit for different reasons. It all starts to go wrong after the witches' prophecy, which predicts not only that Macbeth will be king but also that Banquo's sons will also one day ascend the throne. Though both Macbeth and Banquo are disturbed by the prophecy, Macbeth takes it much more seriously. It exerts a dangerous hold upon his imagination, making him insanely ambitious. As such, Banquo is no longer a friend or a colleague but a rival—a very dangerous rival at that, a threat to be removed.
Once Macbeth has become ensconced upon the throne, Banquo's days are effectively numbered. Macbeth is no longer prepared to listen to advice, whether it's from Lady Macbeth, Banquo, or anyone else. Banquo, for his part, suspects that Macbeth had something to do with Duncan's murder; this makes him even more of a threat. Unlike Macbeth, Banquo understands all too well the dangers of messing around with the forces of darkness. Yet it is precisely those very forces to which Macbeth has succumbed in his seemingly insatiable lust for power. There can, therefore, be no reconciliation between the two former friends, and by the time of Banquo's murder they are sworn enemies.