In Act 2, scene 1, Iago's comment that Othello and Desdemona "are well tuned" is a metaphor of harmonic music in which he uses to indicate the current harmony of Othello's marriage. However the comment is then followed by Iago's vow "to set down the pegs", which show his intentions to disrupt the harmony between them.
When Iago and Roderigo are left alone together, Iago sees this as an opportunity to manipulate Roderigo by telling him that "Desdemona is directly in love with him" (him being Cassio), because she must necessarily tire of Othello. However, although Roderigo finds it impossible to believe, Iago continues by supporting his allegation by overwhelming him with his arguments that Desdemona only fell in love with Othello because of his "bragging" and "fantastical lies" of the stories that he told her about his life.
Iago's argument is sophisticated with the use of rhetorical questions in which he uses with the intent to make an infatuated Roderigo hesitant of Cassio.
"Mark me with what violence she first loved the Moor but for and telling her fantastical lies. And will she still love him for prating?"
"Her eye must be fed. And what delight shall she have to look on the devil?"
"Now, sir, this granted - as it is a most pregnant and unforced position - who stands so eminent in the degree of this fortune as Cassio does?"
The way that Iago uses his rhetorical questions are effective, because he creates a visual before it, in order to make Roderigo think about what Iago is telling him. The rhetorical questions within Iago's explanation make it seem as if he had already planned on what he would say and how he would say it, to give the impression that he isn't misleading Roderigo.
In the quotation: "Her eye must be fed. And what delight shall she have to look on the devil?" Iago compares Othello with the Devil (as they used to be portrayed as black), and tells Roderigo that when Desdemona has had her satisfaction with Othello, she will soon find that he is too old, too "black", and too barbaric to renew her desire and that she will then start looking around for someone else. Iago insists that she will see Cassio, as he is a "pestilent complete knave", who is young and handsome, who women already lust over. The way that Iago constructs his argument is by using the fact that Cassio is a charmer when it comes to women. By describing Cassio as a "knave", Iago is simply saying that Cassio is an unprincipled person when it comes to women.
Iago is clever as he carefully chooses the "right" words when describing Cassio as the description of him being a knave reflects upon the fact that he and Desdemona were holding hands.
However, although Roderigo still refuses to believe Iago and strongly insists that Desdemona held Cassio's hand out of courtesy, Iago's manipulation becomes more forceful.
Iago is desperate to convince Roderigo of Cassio's ill intentions that he tells him:
"They met so near with their lips that their breaths embraced together - villainous thoughts, Roderigo!"
Here, Iago, yet again carefully chooses the words to describe Cassio and Desdemona's actions. He creates a romantic image between them. Iago uses the word "embrace" when describing their breaths, which gives a slight hint of a metaphor of Cassio and Desdemona kissing. The word embrace, itself is a word that is often used to describe something loving.
Iago then uses the advantage of the fact that Cassio and Roderigo do not know each other, to manipulate Roderigo into provoking him:
"Cassio knows you not; I'll not be far from you. Do you find some occasion to anger Cassio, either by speaking too loud or tainting his discipline, or from what other course you please, which the time shall more favourably minister."
To reassure Roderigo, Iago tells him that he will be nearby when he provokes Cassio. He even suggests some ideas of how he should provoke him, which shows the impertinence of Iago.
However, although Roderigo agrees to provoke Cassio, there is still the matter of the fact that Desdemona has rejected Roderigo a long time ago, and had already said that she wouldn't have him if he were the last man on earth, so Iago's plan doesn't make much sense. However yet again, Roderigo is a foolish man and falls into Iago's manipulative plan.
As Roderigo leaves, Iago is left alone again, and explains his actions to the audience in a soliloquy, that he secretly lusts after Desdemona, partly because he suspects that Othello has slept with Emilia, his wife, and he wants to get even with the Moor "wife for wife". As, Iago continues, he states that if he is unable to get his revenge by sleeping with Desdemona, that Roderigo's accusation of Cassio will make surely make Othello suspect his lieutenant of sleeping with his wife and torture Othello into paranoia.
Iago's manipulation of Roderigo seemed easy, as Roderigo didn't really argue back. Yet when he did, Iago still went one step further than him and succeeded by the choice of his words and the tone in which he said them. Iago is clearly a manipulative person and will not stop until he gets his way, as he became quite forceful towards the end of the argument, which shows that he was losing his patience with Roderigo.
In Iago's soliloquy it shows that he is paranoid as he suspects that Othello has slept with his wife Emilia. However, it seems as if he could just be using that as an excuse to ruin the harmony between Othello and Desdemona as he insulted Emilia earlier on. It seems that Iago doesn't care about the after affects of what will happen when his plan fully succeeds, but only of his own intentions, which strongly suggest that he is a very gluttonous person.