Shakespeare's writing tends to have an antagonist who is power hungry. In the play "Othello" the bad guy, Iago, uses his perceptiveness to pick up on Othello's doubts towards Desdemona's faithfulness in order to destroy their relationship. Iago is able to use his people skills to get close to Othello and then use this gained trust in order to devastate him.
In the beginning of the play we quickly discover that Iago only has bad intentions towards Othello, seeing that he tried to demolish their marriage. Since his original idea hadn't worked Iago was working out an immense scheme to break down Othello. Iago is bent on crippling his brave commander for one reason alone, revenge. He always felt as if Othello wasn't worthy of the respect that he had from everyone around him. Iago was jealous of Othello's high position in society, and found that he was not worthy of Desdemona's heart. So Iago was going to do whatever it took to ruin Othello's reputation.
After achieving the position of lieutenant by eliminating Cassio from the picture, Iago began to gain Othello's trust. Iago was very good at manipulating someone by gaining their trust and using words of wisdom to tell people what they want to hear. In act 4 scene 1 specifically Iago begins talking to Othello about the alleged relationship between Desdemona and Cassio. Iago tells Othello some words that Othello wanted to hear, and this made him listen to Iago intently. After Othello was hanging on his every word Iago states, "[Faith,] that he did - I know not what he did."(Act 4 sc. 1 line 38) This makes Othello tense because he wants to hear what Iago know about Cassio. When Othello asks what they did all Iago said was, "Lie." (Act 4 sc. 1 line 40) It was that one small word which drove Othello insane with anger. Iago knew that he couldn't tell Othello everything he allegedly knew about Cassio; otherwise Othello wouldn't be so distraught.