Othello: Dramatic Irony

"IAGO: Ply Desdemona well, and you are sure on 't.
Now, if this suit lay in Bianca's power,
How quickly should you speed!"
pg. 1429

In Act IV, Othello is in hiding while Iago and Cassio are having a discussion. Trying to convince him that Desdemona is unfaithful, Iago tricks Othello into thinking he and Cassio are discussing Cassio and Desdemona's relationship. However, the audience knows that Cassio is really referencing Bianca when he says "I marry her! What, a customer!", basically making fun of the idea of their marriage. Othello, who is subject to only what he hears, which is only about Desdemona, is prone to thinking that Cassio is talking about Desdemona, and he takes this reaction as confirmation that Desdemona is being unfaithful. Much of the play centers around dramatic irony like this, where only Iago and the reader know the real situation. It creates a lot of suspense in the story, wondering how things will play out and if Iago will ever be caught. The audience becomes more personally involved and interested in the story. This sort of secretive method helps to pull the audience into the story so that they don't feel detached from the characters or plot.