Lieutenant.
He is Othello's standard bearer and third ranking officer. Othello as General with Cassio as Lieutenant
Both. In a way. He is jealous of Othello because he has everything he wants: a high rank and a beautiful wife from a upper class family. He is jealous of Cassio because Othello has promoted him to Lieutenant and not him.
They had a good relationship - Othello trusted Cassio and Cassio respected Othello
Cassio was Othello's lieutenant - his second in command.
Lieutenant.
He is Othello's standard bearer and third ranking officer. Othello as General with Cassio as Lieutenant
Both. In a way. He is jealous of Othello because he has everything he wants: a high rank and a beautiful wife from a upper class family. He is jealous of Cassio because Othello has promoted him to Lieutenant and not him.
They had a good relationship - Othello trusted Cassio and Cassio respected Othello
Cassio was Othello's lieutenant - his second in command.
He demotes him. He strips him of his rank as lieutenant and tells him "Never more be officer of mine."
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.
When Iago tells Othello Cassio slept with her.
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.
Cassio and Othello were very close friends before the play.
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.