3 ideas
The Immorality Act and the Population Registration Act turned private facts, such as ancestry or a sexual relationship across the colour line, into criminal liabilities that could destroy a person's life. This creates a market for concealment: whoever knows the secret gains leverage over the person who holds it. The state's own law generates the motive that its police are then sent to investigate.
The novel shows apartheid through paperwork, jurisdiction, classification records and interrogation routine rather than through speeches or spectacular violence. Presented as ordinary procedure, racial domination looks administrative, normal and self-perpetuating. That ordinariness is more damning than overt brutality because it shows how the system ran every day.
Sergeant Zondi can reach Black informants, townships and knowledge that are closed to Lieutenant Kramer, so the investigation depends on him. The law and the police hierarchy still keep him formally inferior. The partnership works through mutual reliance and private respect, while the official structure denies the equality their success depends on.