In act 3, scene 3, Desdemona urges Othello to reenlist Michael Cassio before leaving the scene. Iago then begins asking Othello questions regarding Cassio's role in his relationship with Desdemona. Once Othello admits that Cassio used to carry messages back and forth while he was courting Desdemona, Iago begins acting concerned and suspicious. When Othello asks Iago if Cassio is an honest man, Iago insinuates that Cassio is not to be trusted. Iago does an excellent job of piquing Othello's interest by reluctantly disclosing information regarding Cassio's "true" character. Iago builds suspense by refusing to speak about Cassio's personality, which makes Othello increasingly suspicious. Iago also feigns friendship by warning Othello about becoming jealous while simultaneously planting seeds of jealousy and urging Othello to pay close attention to how Cassio and Desdemona interact.

Iago also reminds Othello that Desdemona has been deceptive before and lied to her father. Iago then reiterates that he only disclosed the unfortunate information because he loves Othello and urges him not to take it too seriously. Before Iago leaves the scene, he manipulates Othello's low self-esteem by discussing Desdemona's desire to be with the young, attractive Michael Cassio. Overall, Iago plants the seeds of jealousy in Othello by insinuating that Cassio is untrustworthy, reminding him of Desdemona's deception, and informing Othello that Michael Cassio is much more attractive than him.

In Othello, Act 3, Scene 3, Cassio visits Desdemona to beg for her assistance in getting his position back. While he is leaving Desdemona, Othello sees Cassio and questions whether or not that it was Cassio:

Wasn’t that Cassio leaving my wife?

At this point, Iago plants a seed of doubt in Othello's mind. He begins to express that Cassio is sneaking away:

Cassio, my lord! No, sure, I cannot believe
That he would steal away so guiltily,
Seeing you coming.

In this manner, Iago is trying to make Othello jealous. He plants seeds of suspicion in Othello. Immediately, Othello becomes suspicious. 

Next, Iago begins to plant seeds of doubt about Desdemona's character. He mentions to Othello that she lied to her father once:

She deceived her father by marrying you;

By this point, Othello is thinking seriously about Iago's accusations. Othello admits that Iago is right:

And so she did.

Now, Othello is becoming jealous. Iago has set the green-eyed monster free. Iago is cunning, crafty. He has planted seeds of doubt and suspicion. Now, Othello is in a position to see what Iago wants him to see. Before long, Othello will be consumed with jealousy until he cannot escape it. Iago has done what he set out to do.