The most detailed interrogations of the novel are those that take place upon Colonel Joll's initial arrival at the town.
Joll tortures an old man and his grandson who have come to town to find a doctor for the young man. He has a sore that will not heal.
Joll believes that the two men are either part of a barbarian vanguard, preparing an assault on the empire, or that they are spies sent to gather information. In discussing this belief, the colonel expresses his philosophy of interrogation and torture; his attitude about his "inquiries into the 'truth'".
He says that truth follows pain. So he inflicts pain and expects that a person's story will change. The magistrate protests, saying that it may be possible that the story changes in order to please the torturer. The truth is exchanged for a lie that might make the pain stop.
Joll scoffs at this notion. The result is that the old man dies with his grandson looking on.