Act II provides a clear delineation of the three characters. It is in this act that we learn what sets the three apart. The act also provides insight into how exactly Othello and Cassio, specifically, become targets for Iago's manipulation.
We discover in scene one that Cassio, Othello's newly-appointed lieutenant,...
has recently set foot on Cyprus and is now, with some trepidation, awaiting the arrival of his master and his beautiful, newly-wed bride, Desdemona. Cassio expresses concern since he had lost contact with the general's ship during an inordinately rough period at sea. He is generous in his praise for Othello and seems genuinely concerned. He has only high praise for Desdemona and is full of complimentaryrhetoric when asked about her. It is clear from his conversation that he has an open and generous nature, one who seems to see only the good in others. We soon discover this naivete is what Iago sees as a weakness which he can exploit.
Next to arrive on the scene is Iago, accompanied by Emilia and Desdemona. Cassio expresses delight at their safe arrival and welcomes them warmly, going so far as to kiss Emilia, Iago's wife.
We are quickly made aware of Iago's cynicism when he makes a derogatory remark about women which leads to Desdemona reprimanding him for his slander. As their conversation continues, Iago makes his misogyny quite clear. His verbal altercation with Desdemona makes her feel somewhat insulted. We also soon learn of his planned malice when he comments on Cassio's warm approach to Desdemona as the lieutenant takes her palm.
[Aside] He takes her by the palm: ay, well said,whisper: with as little a web as this will Iensnare as great a fly as Cassio.
It is obvious Iago has already formulated a pernicious scheme to implicate Cassio for having an illicit relationship with Desdemona and thus make him lose his position. He is bitter and resents that Othello decided to appoint Cassio and not him.
With Othello's arrival, we are made patently aware of the love he and his wife have for each other. Both express relief at seeing one another and Othello, especially, grandly expresses his joy. Once again, Iago proves just how devious and malicious he is by saying, as he witnesses the two lovers' reunion:
O, you are well tuned now!But I'll set down the pegs that make this music,As honest as I am.
The implication is that, in being true to himself, Iago will make it his duty to upset the harmony between the two by letting them play to his tune. Towards the end of Scene 1, Iago further illustrates his demonic nature by convincing Roderigo that Desdemona is in love with Cassio. At the conclusion of the scene, he declares his evil intent to mislead Othello and manipulate him into believing that he, as his ancient, truly only has the general's best interests at heart.
In scene three, Iago's attempt to manipulate Cassio into confessing that Desdemona is a flirt fails miserably, as the young lieutenant only has good to say about her. He then persuades the somewhat gullible officer into drinking some wine, something the lieutenant acknowledges he is not used to because he is easily inebriated. It is clear Cassio wishes to please Iago and probably acquiesces to his request to avoid offending him, unaware he is being manipulated.
Further proof of Iago's malcontent is given when he uses Roderigo to set up an altercation with Cassio, who has become tipsy and, therefore, forgetful of his place and duty. The altercation turns into a brawl, which draws Othello's attention. True to character, Iago, "in all honesty" makes a plea in Cassio's defense, and, in an act of deliberate irony, points a finger at the embarrassed lieutenant for being responsible for the fracas. Othello immediately dismisses Cassio.
Othello's actions in this scene confirm his status as a leader. He quickly intervenes and metes out punishment. He is harsh, uncompromising, and considerate of the confusion and anxiety that any disruption might bring to the already-fearful citizens of Cyprus. Cassio is deeply humiliated by the dismissal and, to deepen the irony, turns to Iago for advice. The sly ancient suggests the embarrassed ex-lieutenant approach Desdemona for assistance.
The scene ends with Iago expressing probably his darkest desire: he is about to begin a campaign in which he will absolutely and irrevocably demonize the innocent and virtuous Desdemona.
So will I turn her virtue into pitch,And out of her own goodness make the netThat shall enmesh them all.
He is prepared to sink to the depths of perversion and destroy innocent lives, not only because he wants to get back at the general but also because he hates everything that is good and pure. In this way, Iago proves he is the epitome of evil.