Most of the dramatic irony in Othello comes from Iago. We, the audience, know that he's a thoroughly nasty piece of work, hell-bent on destroying the man who's supposed to be his master, but none of the characters on stage know this until it's too late, and all the damage has been done. Initially, as far as they're concerned, Iago is as honest as the day is long and every bit as noble as he likes to make out.
Of all the characters, it's Othello who's most taken in by Iago's devious wiles. A prime example of this comes in Act 3, Scene 3, when Othello expresses his confidence in Iago's honesty:
I know thou'rt full of love and honesty
And weigh'st thy words before thou giv'st them breath
We, on the other hand, know better. We've already heard Iago openly express his hatred for the Moor. We've seen him in action as he goes about plotting and scheming to make Othello think that Desdemona's been cheating on him with Cassio. So, in a classic example of dramatic irony, we know something that a...
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