Banquo is an important character because he begins his place in the story running parallel with macbeth, they are both worthy Thanes, both great warriors and both 'loyal' to the King Duncan. Most importantly they encounter the withches and their prophecies.
Banquo is promised a part of the prophicies, but his reaction to the witches is different to Macbeths. Banquo challenges them ".are you aught that man may question?" Banquo is puzzeled by how Macbeth reacts "Good Sir, why do you start and seem to fear things that do sound so fair?" .
Unlike Macbeth he is not wrapt withall, but seems calm, purposeful and matter of fact he actually appears to be more interested in what exactly the witches are "What are these, so wither'd and so wild in there attire. That look not like th' inhabitants o' th' earth. And yet are on't?.
The character of Banquo undergoes some change. He becomes troubled and edgy. Like Macbeth he no longer sleeps. He has obviously been struggling with temptation. The banquo who seemed unmoved by the withches is now uneasy. He tells Macbeth "I dreamt last night of the three weird sisters" However Banquo openly admits his temptations whereas Macbeth pretends not to think of them "i think not of them".
If theres a weakness in Banquos character it might be his failure to act. Why does he not reveal what he knew about the prophicies? We never found out because Macbeth became suspicious of what Banquo knew and gets him killed.
In conclusion it is clear to say Banquo knew a lot but did nothing!.