Iago is going to talk to Cassio about Bianca. Othello will see this interaction and will misinterpret the conversation thinking they are talking about Desdemona, which will mad him mad.
"Cassio's smiles, gestures, and light behaviors quite in the wrong.PoorAs he shall smile, Othello shall go mad, and his unbookish jealousy must construe."
Bianca confronts Cassio about the handkerchief that he gave to her. Othello sees that it is his handkerchief that he gave to Desdemona, likely thinking that she gave the handkerchief to him.
"How now, my sweet Bianca? How now? How now?"
"Let the devil and his dam haunt you!..I was a fine fool to take it!...This is some minx's token, and I must take out the work!.."
Desdemona tries to reply to Lodovico about the relationship between Cassio and Othello, which makes Othello mad. Othello then slaps Desdemona in front of Lodovico.
"What, is he angry?"
"Are you wise?"
"Fire and brimstone!"
"A most unhappy one. I would do much t' atone them, for the love I bear to Cassio."
Iago and Emilia try to talk to Desdemona about how Othello has been treating her. Iago says that some trick must have caused this, but really it's just Iago getting into Othello's head with all these scenarios. Emilia has no idea that Iago is doing this.
"Beshrew him for't! How come this trick upon him?"
"Nay, heaven doth know."
"I will be hanged if some eternal villain, some busy and insinuating rogue, some cogging, cozening slave, to get some office, have not devised this slander. I will be hanged else."
Rodreigo confronts Iago about how his money isn't providing results. Iago then says that to get Othello and Desdemona to stay in Cyprus a bit longer, he must kill Cassio so he can't take Othello's place. Iago convinces Roderigo that he needs to kill Cassio.
"Unless his abode be lingered here by some accident-wherein none can be so determinate as the removing of Cassio."
"How do you mean, removing him?"
"Why, by making him uncapeable of Othello's place: knocking out his brains."
Desdemona talks to Emilia about if she dies, she wants to be wrapped in wedding her bed sheets . Foreshadowing that she might die.
"ll's one. Good faith how foolish are our minds! If I do die before thee, prithee, shroud me in one of those same sheets."