"Master of the House"
I've heard it said that the film doesn't make the most of M. Threnadier, other than having fun with his big song. Some people have complained that Cohen wasn't nearly as funny or charming as some stage versions, but I think they're wrong to carp. Not about missing opportunities for maximum laughs, which is true, but about making the character as amusing as possible.
This story is a tense drama above all, and when he reappears in the second act the audience should cringe and say "Oh no, it's the sociopath who sells children!", not "Hey, it's the fun guy!". Funny he may be, but he and his wife are monsters.