Iago first manages to convince Othello that he is trustworthy. He shows his contempt for Othello when he says in Act I that he will lead Othello tenderly by the nose, "as asses are." We see that Iago's strategy is working by Act II, for Othello says of this most deceitful man: "Iago is most honest."
Because Iago has won Othello's trust, he is able to cast aspersions on others. He says in Act III, with complete cynicism, "men should be what they seem," through this implying that Cassio might be involved with Desdemona.
Iago works relentlessly throughout the play to destroy Othello's happiness. Sensing Othello's insecurity as a middle-aged black man married to a much younger woman, and projecting his own conviction that women are inherently unfaithful, he is able to use suggestion and innuendo to lead Othello to the false conclusion that Desdemona has betrayed him.
One way he does this is to contrive to have Othello overhear Cassio deriding his mistress, Bianca, while leading Othello to believe Cassio is talking of Desdemona. But, in actuality, it is not one incident, but a steady drip of Iago's falseness and innuendo that weaves a web of deceit around Othello.
Act 3 Scene 3 is known as “the seduction scene” (or “the manipulation scene”) because here Iago tricks Othello into believing that Desdemona is having a love affair with Cassio. Iago and Othello enter just before Cassio leaves, hearing the tail-end of the conversation. Iago says, “Ha! I like not that.,” which triggers a back and forth repartee between Iago and Othello that is like a dance: Iago drops a hint or uses a tone suggesting a relationship between Desdemona and Cassio, Othello asks what he means by the comment, Iago then demurs, only for Othello to demand more information, seemingly dragging it out of Iago while all the while it is Iago leading this dance. Iago’s skills at manipulation result in tremendous irony, for we the audience know what Iago is up to, but Othello seems like a dupe in not understanding what seems to us obvious manipulation.
Like a true evil genius, Iago plays upon Othello's own fears and reinforces those fears with lies and innuendo (hints). Iago manipulates the situation so that Cassio is in a position to ask Desdemona for aid. He then stands by Othello's side, professing concern for his friend, and questioning Desdemona's fidelity because she spoke for Cassio. By stealing Desdemona's handkerchief, Iago is able to plant it on Cassio and provide evidence for his lies. Finally, Iago arranges for Othello to overhear a conversation between himself and Cassio so that Othello believes he is hearing a confession. It all works. Othello gives in to his fears and his natural jealousy and he kills the woman he loves.
Most blame Iago for his duplicitious behavior, but some blame should also be laid upon Othello, who chose not to handle the situation with calm reason.
by lying throughout