Cassio thought Iago was giving him a good idea but Iago was planning to convince Othello that Desdemona was having an affair with Cassio. Every character thinks Iago is a good nature and honest person but he is a manipulator and snake. Cassio goes to Desdemona and tells her he is sorry and his whole situation. After, Cassio tells Desdemona his problems as soon as Cassio sees Othello he leaves but Desdemona tries to convince him to say. Desdemona tries to convince Othello to take back Cassio and have him as a friend again but he brushes her off and leaves. Iago acts like he just wants to have a conversation but he dropping unusual hints to Othello which gets him interested in 3.3.163-165.
Othello says he is not the jealous type but even this type of information show what Iago said has hit a nerve. Iago is controlling everybody through his words and manipulation in all three of the acts. Othello even though he is not a jealous man probably do believe what Iago says. Everybody claim Iago is this good and honest person but he is very evil and a snake to his society. Othello and Cassio never talked after he was demoted. A lot of the information that is passed around is through somebody else instead of face to face contact. Iago is showing how to gain power and control through this whole