In Othello, the audience is first introduced to Iago and has already established his cunning and his intention to "serve my turn upon him."(I.i.42) When Othello is initially made known, the audience already has an image, due to Iago's crude descriptions, of an "old black ram"(89)and a "Barbary horse"(112) The audience is aware, from Iago's interference, that it is Othello who has apparently poisoned Desdemona with "spells and medicines." (I.iii.61) Brabantio explains the problem to the Duke of how Desdemona is effectively "dead" to him without mentioning the reasons and the Duke is shocked. The audience already waits in anticipation for what will follow. The Duke's statement is purposeful and determined as he explains that:
"Whoe'er he be that in this foul proceeding Hath thus beguiled your daughter of herself, And you of her, the bloody book of law You shall yourself read in the bitter letter After your own sense." (I.iii.65-69)
Hence the audience knows that misunderstandings will abound as appearances deceive. Iago has made sure that the audience knows that, in his own words, "I am not what I am." (I.i.66)
Dramatic irony allows an audience to have an understanding of a situation which escapes the actual characters themselves. This is certainly the case as the Duke is unaware at this stage who it is that has so enraged Brabantio and is affected by the intensity of Brabantio's behavior and own reaction. This creation and use of dramatic irony therefore foreshadows events that will take place and the unfortunate turn of events leading to Othello's downfall.