In Act 3, Iago and Othello discuss the possibility of Desdemona's unfaithfulness. Othello does not believe Iago's claims at this point; however, he is becoming suspicious and worries that the rumor might in fact be true. Iago tells Othello that Cassio is an honest man:
Men should be what they seem. Or those that be not, would they might seem none. (III.iii.124-125)
This is ironic because Iago does not want Othello to believe that Cassio is an honest man--he is simply planting a seed of doubt in Othello's mind. Further, Iago suggests that a man's true character should be what he shows other people; however, this is certainly not the case for Iago who is scheming under Othello's nose.