Cassio sees Desdemona as a sort of secular Virgin Mary, as from the quote, "O behold...Hail to thee lady!". On the other hand, Iago sees Desdemona as competition for Othello's affections.
Iago advises Cassio to ask Desdemona to plead on his behalf. This will help Iago poison Othello's mind, as Desdemona will appear to be doing so out of her (illicit) love for Cassio.
He tells Cassio to ask Desdemona for her help to get him reinstated.
Iago says that he was sleeping next to Cassio and that Cassio was having a dream about making love to Desdemona.
Iago tells Cassio that he must persuade Othello to reinstate him. He says "the general's wife is now our general" and so the best way to persuade Othello is to get Desdemona to plead on Cassio's behalf. Since Iago has also told Othello that Desdemona and Cassio are having an affair, the harder Desdemona pleads, the worse it is going to look for both her and Cassio.
While Othello is hiding, Iago and Cassio discuss Desdemona. Iago uses this opportunity to manipulate Cassio into speaking favorably about Desdemona, knowing that Othello is eavesdropping. This conversation plays a crucial role in Iago's scheme to incite jealousy in Othello regarding Desdemona's loyalty.
Iago advises Cassio to ask Desdemona to plead on his behalf. This will help Iago poison Othello's mind, as Desdemona will appear to be doing so out of her (illicit) love for Cassio.
He tells Cassio to ask Desdemona for her help to get him reinstated.
Iago says that he was sleeping next to Cassio and that Cassio was having a dream about making love to Desdemona.
Iago tells Cassio that he must persuade Othello to reinstate him. He says "the general's wife is now our general" and so the best way to persuade Othello is to get Desdemona to plead on Cassio's behalf. Since Iago has also told Othello that Desdemona and Cassio are having an affair, the harder Desdemona pleads, the worse it is going to look for both her and Cassio.
Iago realizes that Cassio greets Desdemona romantically, and with a kiss on the cheek. Supposably that's how you greet the women politely.
Othello was just out of earshot when Iago talks to Cassio. He has told Othello that he is talking to Cassio about Desdemona but he is really talking about the woman that Cassio is really having an affair with, Bianca. In 'Othello' Iago tricks Othello by planting Desdemona's handkerchief in Cassio's room.
Desdemona. Iago says that Desdemona is the most influential person with Othello, so Cassio should get her to plead on his behalf. This sounds reasonable, except that Iago has suggested that Cassio and Desdemona are having an affair, and this will heighten Othello's suspicions.
If Desdemona asked a favour on Cassio's behalf, it would look to Othello like she was doing it out of an improper love for Cassio. The harder she argues, the more suspicious it seems to Othello.
Primarily the fact that Iago has suggested to him that that is what is being talked about. By this point, Iago has got Othello into such a state he cannot interpret anything Cassio or Desdemona does as being innocent.
Iago sets out to trap Othello by several means. He persuades Desdemona to plead for Cassio and persuades Othello that Desdemona pleading for Cassio proves she is in love with him. He steals the handkerchief, persuades Othello that she has given it away to Cassio, then gives it to Cassio himself. He gets lucky as Cassio gives it to Bianca and she returns it in Othello's sight. He talks to Cassio about Bianca, telling Othello who can see (but not hear) that he is talking about Desdemona. All of this circumstantial evidence bolsters the outright lies that Iago tells about Cassio admitting to having an affair with Desdemona.
Cassio kisses Desdemona's hand. Iago says, " He takes her by the palm: ay, well said, whisper. With as little a web as this will I ensnare as great a fly as Cassio."
It would make it seem that Iago did his part in the plan, where Iago kills Cassio and Othello kills Desdemona. Even though Iago got Roderigo to do it for him.