The reader or audience knows something the character does not, and so the character’s words and actions take on a meaning he himself does not perceive.
In Macbeth
If a man were porter of hell-gate, he should have old turning the key.
The Porter jokes that he would be kept turning the key for all the people who arrive at the "Gates of Hell," without knowing what the audience already knows after the previous scene. Without meaning to, he hits on what the castle has become.
Act II: The Murder of Duncan, the Porter
'Gainst nature still! / Thriftless ambition, that will ravin up / Thine own life's means!
Act II: The Murder of Duncan, Ross
I drink to th' general joy o' th' whole table, / And to our dear friend Banquo, whom we miss. [...] Avaunt, and quit my sight! Let the earth hide thee! / Thy bones are marrowless, thy blood is cold; / Thou hast no speculation in those eyes / Which thou dost glare with.
Act III: The King Ruled by Fear, Macbeth
Let every soldier hew him down a bough / And bear 't before him. Thereby shall we shadow / The numbers of our host and make discovery / Err in report of us.
Act V: Sleepwalking, Birnam, and the Fall, Malcolm
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