The role is thought to have been first played by Robert Armin, who typically played intelligent clown roles like Touchstone in As You Like It or Feste in Twelfth Night.
The character's source is traced to Giovanni Battista Giraldi Cinthio's tale "Un Capitano Moro" in Gli Hecatommithi (1565). There, the character is simply "the ensign".
He manipulates his wife Emilia, Desdemona's lady-in-waiting, into taking from Desdemona a handkerchief that Othello had given her; he then tells Othello that he had seen it in Cassio's possession. Once Othello flies into a jealous rage, Iago tells him to hide and look on while he (Iago) talks to Cassio. Iago then leads Othello to believe that a bawdy conversation about Cassio's mistress, Bianca, is in fact about Desdemona. Mad with jealousy, Othello orders Iago to kill Cassio, promising to make him lieutenant in return. Iago then engineers a fight between Cassio and Roderigo in which the latter is killed (by Iago himself, double-crossing his ally), but the former merely wounded.
Iago’s plan appears to succeed when Othello kills Desdemona, who is innocent of Iago's charges. Soon afterwards, however, Emilia brings Iago’s treachery to light, and Iago kills her in a fit of rage before being arrested. He remains famously reticent when pressed for an explanation of his actions before he is arrested: "Demand me nothing. What you know, you know. From this time forth I never will speak word." Following Othello's suicide, Cassio, now in charge, condemns Iago to be imprisoned and tortured as punishment for his crimes.
Iago is one of Shakespeare's most sinister villains, often considered such because of the unique trust that Othello places in him, which he betrays while maintaining his reputation of honesty and dedication. Shakespeare contrasts Iago with Othello's nobility and integrity. With 1,097 lines, Iago has more lines in the play than Othello himself.
Iago is a Machiavellian schemer and manipulator, as he is often referred to as "honest Iago", displaying his skill at deceiving other characters so that not only do they not suspect him, but they count on him as the person most likely to be truthful.
Shakespearean critic A. C. Bradley said that "evil has nowhere else been portrayed with such mastery as in the evil character of Iago", and also states that he "stands supreme among Shakespeare's evil characters because the greatest intensity and subtlety of imagination have gone into his making" The mystery surrounding Iago’s actual motives continues to intrigue readers and fuel scholarly debate.