They had a good relationship - Othello trusted Cassio and Cassio respected Othello
Cassio was Othello's lieutenant - his second in command.
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.
When Iago tells Othello Cassio slept with her.
Cassio and Othello were very close friends before the play.
They had a good relationship - Othello trusted Cassio and Cassio respected Othello
Cassio was Othello's lieutenant - his second in command.
Emilia overhears Othello and Desdemona discussing Cassio. Desdemona is pleading on Cassio's behalf to Othello, hoping to persuade him to reinstate Cassio as his lieutenant. In the conversation, Desdemona is trying to help Cassio regain his position in Othello's esteem.
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.
When Iago tells Othello Cassio slept with her.
Cassio and Othello were very close friends before the play.
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.
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.
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.
Lieutenant.
He told Montano that Cassio was drunk every night. He also casted doubt on Othello's judgment for appointing Cassio, who might be drunk in a moment of crisis.
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.