How do the less important yet very important characters like Duncan, Macduff, Banquo and Malcolm, help clarify the character of Macbeth?
They are foils...each of these characters by comparison to Macbeth brings out the obvious flaws and faults that Macbeth embodies.
Macbeth-Banquo--together they are invincible warriors who overthrow Macdonwald and the other traitors. They are both noble, honorable and worthy of reward. The witches tell us that Banquo is lesser but more; happier, but less happy than Macbeth. This begins to shape the characters--Banquo is lower in status but more secure in morals and values. Therefore he is happy in himself for eternity if not for the short term.
Duncan-Macbeth--As far as rulers go, Duncan is kind, noble, and well-loved by his people. Even the earth rebels on the night of his murder. Macbeth is the stark opposite as a ruler. No one trusts him, many suspect him, warriors abandon him, the earth does not respond when he is overthrown.
Malcolm is a chip off the old block--Duncan. He is honest, noble, and the rightful heir to the throne. He brings peace back to Scotland as his father before him. Men sacrifice everything (Macduff) to support him for love of Malcolm and of Scotland.
Through the characters' words, actions, and how they interact with one another, they help develop Macbeth's character by representing different ends of the spectrum. Good vs. Evil is much more obvious when you compare these minor characters with Macbeth's track record. It is easier to track when and where Macbeth fell.