The reader or the audience knows something that the character ignores, and so the character's words and acts take on a meaning that he himself does not perceive.
In Othello
O, beware, my lord, of jealousy! / It is the green-eyed monster, which doth mock / The meat it feeds on.
It is Iago, who is manufacturing the jealousy, who warns Othello against it. The audience knows this, and so the "Beware, my lord" sounds like a friend's advice when it is, in fact, part of the trap.
Act III: The temptation and the handkerchief, Iago
Sweet Desdemona, / Let us be wary, let us hide our loves
Act III: The temptation and the handkerchief, Iago, recounting Cassio's supposed dream
The handkerchief!
Act III: The temptation and the handkerchief, Othello
Your wife, my lord; your true / And loyal wife.
Act IV: Humiliation and the willow song, Desdemona
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