Keep in mind that Brandt and Father aren't taking Prozium. They--most likely along with a small group of elites--enjoy a privileged yet secretive status with access to art, pleasure and expression. Brandt is clearly adept at hiding his emotions but cannot help but gloat when he deceives Preston, the master cleric capable of sniffing out sense offenders with only the smallest clue.
Now, why would the regular non-cleric cops tolerate these displays of emotion? First, they were only responsible for enforcing the rules of the clerics. When Preston is caught outside the city (when he rescue's the dog), the cops withdraw when they realize he's a cleric. It's not until they hear the dog that they intervene. My guess is that the mystery surrounding the clerics and their abilities makes them out of reach of the cops unless another cleric, Brandt in this case, makes the call.