Banquo, in Macbeth, is a character similar in some aspects to Macbeth. While Banquo believes in the prophecies, much like Macbeth, he does not take action to fulfill them.
Banquo sees that Macbeth's prophecy has come true. He, therefore, trusts in the fact that his will as well (his sons will gain the throne). Fortunately for Banquo, he does not take any actions to insure the prophecy. He believes that if they are meant to come true, they will on their own. Internally, Banquo must face the fact, internally, regarding the person that Macbeth has become because of the prophecy. He, not willing to become something else, trusts in fate.
Externally, Banquo is Macbeth's antagonist (in the form of the ghost). Banquo, for Macbeth, represents the good. Banquo must make the decision to act virtiously. He chooses, in the face of Macbeth's murderous actions, to remain vigilant and noble. The external forces brought upon him are Macbeth's actions and the prophecy of the witches. Banquo must face these challenges while remaining a good person.