This grew; I gave commands; Then all smiles stopped together. (45-46)
Not only does the Duke have the power to order someone killed, he doesn’t do his dirty work himself. He only has to give "commands" – he can just pick up the red phone and things get done. This emphasizes how far up the social ladder he is – but it also suggests that he’s dependent on underlings. We’re not sure the snobby Duke has it in him to kill somebody with his own two hands.
Nay, we’ll go Together down, sir. (53-54)
The Duke is obsessed with power in every relationship – not only in a major relationship like his marriage, but also in the minor relationship between him and his listener, the servant of the Count. When the servant tries to get away from him after hearing the story of the Duchess, the Duke insists that they head back to the party together. Not only does this show how the Duke can control every little move the servant makes, it prevents the servant from telling the Count what he’s heard privately – which might make the Count back out on the marriage between his daughter and the Duke.