The reader or audience knows something the character ignores, so the character's words and actions take on a meaning he himself does not perceive.
In The Slum
Era uma comissão de abolicionistas que vinha, de casaca! trazer-lhe respeitosamente o diploma de sócio benemérito, It was a commission of abolitionists coming, in tailcoats! to respectfully bring him the diploma of honorary benefactor member
The reader knows about the knife and the covered face in the shop, and the commission, "in tailcoats", knows nothing: for those who have read the novel, the visitors' respectful words take on a meaning they themselves do not perceive.
Chapters XXI to XXIII: Bertoleza's End and Romão's Rise, the narrator