as her rejected suitor.
In act 1, scene 1, Iago exploits what he knows is Roderigo's passion for Desdemona and his jealousy of Othello to have Roderigo denounce Othello to Desdemona's father, Brabantio, and to alarm Brabantio with news that Othello and Desdemona have eloped.
In act 5, scene 1, Iago again exploits Roderigo's jealousy, but this time it's Roderigo's jealousy toward Cassio, who Iago intimates is Desdemona's lover.
In fact, Iago has long been resentful of Cassio for having been chosen over Iago as Othello's lieutenant, and Iago wants to take revenge against Cassio. Iago also wants to ingratiate himself to Othello by having Cassio killed for Othello's jealousy of Cassio—jealousy that Iago inflamed in Othello by leading Iago to believe that Desdemona has been unfaithful to Othello with Cassio.
Iago induces Roderigo to attack Cassio, seemingly on Roderigo's own behalf but actually to fulfill Iago's evil intentions toward Cassio and Othello.
In each scene, Iago serves as Roderigo's "second," his back-up and accomplice, in order to put the responsibility for success or failure of the schemes against Othello and Cassio on Roderigo and to make sure that Roderigo carries out the schemes to Iago's satisfaction.
As it turns out, Roderigo is fairly incompetent in both schemes, and Iago has to step in to prevent both of his own schemes from failing.
A significant difference between these scenes is that Iago stays in the background in act 1, scene 1, to convince Brabantio of Othello's supposed lustful intentions toward Desdemona. Brabantio doesn't recognize Iago, and Iago purposely leaves the scene before he can be identified as Othello's ensign or be exposed in the plot against Othello.
In act 5, scene 1, Roderigo makes a mess of the planned attack on Cassio, and Iago has to step in to make things right. Roderigo fails to kill Cassio and is instead wounded by Cassio. Iago wounds Cassio from behind, unseen, and runs off. Iago later kills Roderigo because Iago fears that Roderigo will expose Iago's role in the attack on Cassio and his schemes against Cassio and Othello.
Unlike in act 1, scene 1, when Iago leaves the scene so he wouldn't be recognized, in act 5, scene 1, Iago returns to the scene because he needs to control the information. Early in the scene, Iago tells Roderigo that the attack on Cassio "makes us, or it mars us" (5.1.4). Iago must return to the scene to ensure that it doesn't "mar" him.
Iago orchestrates how the events are perceived, gives aid to Cassio, casts suspicion on other characters—including Roderigo and Bianca—and sends Emilia to Othello and Desdemona to tell them what Iago wants them to know.
Another difference between the scenes is that Othello, the object of Iago's scheme in act 1, scene 1, doesn't appear in the scene. Roderigo, Iago, and Brabantio simply talk about him.
In act 5, scene 1, however, both of Iago's victims, Cassio and Othello, appear in the scene. Although Cassio isn't killed, Othello thinks he is, and Othello sets off to fulfill Iago's scheme against him by killing Desdemona, then kills himself when he discovers the truth of the matter.