What do Iago's monologues in Act I scene I of Othello reveal about his character?

Iago's monologues in Act I, Scene I of Othello set up his character as the antagonist of the play. He outlines one of his reasons for hating Othello: the fact that he was passed over for promotion in favour of Cassio, though whether this is his true motivation or not is up to the audience to decide. It becomes clear that Iago is jealous of Cassio's status, one aspect of the important theme of jealousy in the play. In the second monologue, Iago outlines his attention to work against Othello while appearing to serve him. His almost oxymoronic statement "I am not what I am" introduces the deception which is at the heart of Iago's character. By allowing Iago these monologues, Shakespeare introduces an element of dramatic irony, in that the audience knows Iago's plans though the other characters are thoroughly taken in by his deceptions. The space which these monologues allows Iago in order to express himself, especially since they occur before the protagonist of the play has even been introduced, make it clear that Iago will be an important presence in the play: ultimately, he is a major factor in Othello's tragic downfall.