Act III, scene iv assumes the bizarre shape of a perverted trial. From the moment he enters, Othello plays the role of the prosecutor, demanding that Desdemona produce the handkerchief and accusing her of being a whore. Instead of defending herself against her husband’s accusations, Desdemona responds by advocating Cassio’s case, appealing to Othello as a judge of Cassio’s character. The result is a shouting match, wherein husband and wife completely fail to communicate, Othello repeatedly screaming “The handkerchief!” while Desdemona enumerates Cassio’s noble qualities, all of which Othello takes as testimony against her. He points to her moist hand as evidence of her inherently lascivious nature. Finally, the handkerchief itself is the strong circumstantial proof that Iago promised him.
By this point, the plot unfolds without any further assistance from Iago, although he is still involved in manipulating it in some way. He has thus far been so careful to inform the audience of his every plan that it seems like he must have anticipated every turn in the road. As with the characters onstage, Iago’s power with the audience lies in his ability to make them believe he knows more than he does.