Strange that Cassio would guiltily flee from your wife when seeing you coming.
I would prefer not to speak of this now.
You should speak to Cassio, maybe tomorrow or the next day or the next day.....
I demand to know what you are hinting at.
I'd rather not tell you........but
As Cassio uncomfortably leaves when Othello and Iago enter, Iago makes a comment about how quick Cassio is to leave. He insinuates that Cassio is uncomfortable around Othello and a bit too comfortable around Desdemona. This is the start of Iago's mind games with Othello. He hints at things making Othello confused and wanting to know more.
How could she betray me like this?
What's his problem?
The start of the end of the great Othello....
Here Desdemona makes a case for Cassio as she had previously promised she would do for Cassio. Though Othello is not interested and essentially wants to be left alone. She keeps badgering Othello, but he stays calm.
I have found Desdemona's handkerchief!
Finally the last piece of the plan I needed.
You finally did something right!
I demand Cassio's death!
Here we see the beginning of Iago's evil plan against Othello. He starts to tell him of an affair between Desdemona and Cassio and that is why she is vouching for Cassio. He is messing with Othello's mind to get him to enact revenge upon his wife.
Anything for you my lord!
Here Desdemona reenters and notices her husband looks sick. She tries to soothe him, but he resists her due to his new knowledge of her alleged affair. As she walks out she drops a handkerchief that Othello had given to her as a symbol of their love. We see Iago playing with Othello's mind here. Othello starts to doubt his trust in his wife and is putting his full trust in Iago.
Emilia finds the Handkerchief and proudly gives it to Iago. By doing this she is giving him the ammunition to complete his evil plan. She does not know what it is for, but gives it to her husband in an attempt to gain his respect.
Iago shows Othello the handkerchief and says he saw it in the possession of Cassio. This is the final straw for Othello. He declares that Iago must kill Cassio. Iago complies as his plan is now fully in effect.