1.1 Roderigo freaks out when he realizes that Desdemona is marrying Othello instead of him. He blames Iago for not realizing this earlier. Iago tells him it's not over yet; they wake up Desdemona's father and tell him he'd better find his runaway daughter.
1.2 When the Duke approves of Othello and Desdemona's marriage, Roderigo says he'd better give up and drown himself. Iago convinces him to get more money, under the pretense that he can seduce Desdemona much more easily once she's Othello's desperate housewife.
2.1 Iago convinces Roderigo that Desdemona is already getting it on with Cassio, and that he has to beat Cassio up since he's the competition.
2.3 Roderigo gets in a fight with drunken Cassio, but only manages to get beat up himself. He talks about giving up and going back to Venice, but Iago persuades him to keep trying.
4.2 Roderigo finally shows some spine. He accuses Iago of not being up front with him, and he's suspicious that Desdemona has not received all the jewels he sent her. He threatens to make Iago pay if he doesn't produce results ASAP. Iago, however, just praises Roderigo for his boldness and tells him he needs to kill Cassio if he wants to take his place in Desdemona's bed. Once again, Roderigo is persuaded.
5.1 Roderigo tries to kill Cassio and fails, so Iago stabs and kills Roderigo. This way, he won't be able to betray their scheming. Iago is a "I don't want any loose ends" kind of guy.