The reader or audience knows something the character does not, and so the character's words and deeds take on a meaning he himself does not perceive.
In the Iliad
one thing the Father gave him, another he refused with a nod: to keep war and battle away from the ships he granted; to return safe and sound from the battle, he refused.
The narrator tells the reader that Zeus refused Patroclus' return, but Achilles does not hear the refusal and keeps waiting for his friend safe and sound. The difference between what the reader knows and what Achilles ignores makes each of his waits a vain one.