Jealousy and trust, or mistrust, are some of the main themes in Othello. In the first two acts, Othello is dedicated to Desdemona. In Act 3, Scene 3, Iago really begins to prey on Othello's insecurities and plants the seeds of jealousy in Othello's mind. Cassio had met with Desdemona unbeknownst to Othello. Othello questions Desdemona about this and then asks for time to be alone. It is following his conversation with Iago that Othello's jealousy increases. Othello pleads with Iago to tell him what he thinks. Iago is sly in guarding his thoughts, claiming his freedom to do so. Iago also "jealously" guards his own reputation. Iago warns Othello that his thoughts might be "vile and false." This plays on the theme of jealousy but also increases Othello's curiosity and suspicion about what Iago thinks of Desdemona's and Cassio's meeting.
After Iago claims to keep his thoughts to himself and ensure his reputation, he even warns Othello about the dangers of jealousy:
O, beware, my lord, of jealousy.
It is the green-eyed monster which doth mock
The meat it feeds on. That cuckold lives in bliss
Who, certain of his fate, loves not his wronger. (III.iii.169-72)
Othello still needs proof of Desdemona's infidelity. Iago provides this proof (Desdemona's handkerchief) and Othello vows to take revenge on Cassio. Even Bianca, upon finding the handkerchief, becomes jealous. Iago uses other characters' jealousies to position them against each other. As emotions run higher, the drama increases and Othello resorts to violent and drastic actions.