How do the minor characters bring out the themes in Woyzeck?

First, let's talk the major themes. The play is mostly about the cruelty that man does to man, both through inherent qualities and through decisions for the sake of profit. The poverty that traps Woyzeck and Marie cannot be discounted as part of what drives him mad and what drives Marie into the arms of someone from higher class. But more than that, people are generally cruel in Buchner's view and this cruelty necessarily leads to violence. We see this echoed in almost all situations and characters. Marie is not a "bad" person but she has been ruined by poverty into feeling trapped, and so commits her transgression. The Drum Major and Seargent both help illustrate how meaningless is the place of a low soldier in the arm, another stage of Woyzeck's degradation. The policeman at the end shows little concern for the tragic end, calling it "lovely," showing how much violence plays a role in the human sphere. I would say use this type of thought and you can find illustrations of these themes in pretty much every character and they way they treat one another.