When every sentence you speak starts and ends with "Heil Hitler," you're just not going to be the life of the party.
Herr Zeller, the head of the local Nazi organization, is probably the least likeable character in the whole film—he even has the bad guy pencil mustache to go with his nasty attitude. He's written almost as a caricature of the superior, heartless, power-mad, mindless Nazi robot. His primary role in the story is to harass and threaten Captain von Trapp and his family about the need to cooperate with the Nazis as they become the ruling power in Austria.
Zeller can't stand it when he attends a ball at Captain von Trapp's and sees the Austrian flag prominently displayed. He mocks the captain's refusal to admit that Austria is about to become part of Germany. Then, after the Anschluss (or annexation) has indeed occurred as predicted, Zeller takes it upon himself to hang the Nazi flag outside the captain's house while Georg and Maria are on their honeymoon.
It's Extreme Makeover: Nazi Edition.
HERR ZELLER: Heil Hitler. I've come from Captain von Trapp's house. The only one in the area not flying the Third Reich flag. But we have dealt with that.
Zeller communicates the Reich's plans to have the captain take a commission in the German naval forces. He delivers the telegram with the good news, and makes sure it's clear that the captain doesn't have a choice in this decision.
He's a step ahead of the von Trapps when they try to sneak out of Salzburg, and insists on being glued to them for the rest of the evening to ensure that the captain eventually makes his way to Bremerhaven to assume his duties. The captain convinces him that they're just on their way to the festival, and Zeller lets them go.
HERR ZELLER: You will sing. You will all sing. But only because that's what I want. It will demonstrate that nothing in Austria has changed. And when you have finished singing, you, Captain von Trapp, will be taken to Bremerhaven. Now, if you will all get into your car. . .we will escort the von Trapp Family Singers to the festival.
"But only because that's what I want." Just in case the captain gets any ideas that he changed Zeller's mind.
Shmoop enjoys thinking about the look on Zeller's face when he hears the Austrian audience joining in as the von Trapp family sings "Edelweiss." We enjoy even more imagining what his superiors will do to him when they hear about how the family escaped on his watch.