When evaluating whether or not the success of the play Othello lies in Shakespeare's crafting of the character of Iago, consider how Iago certainly stands out as a strong antagonist.

First, think about the characteristics Iago exhibits and how they contribute to the play and its plot. You might talk about Iago's extreme jealousy and clever manipulation. You could discuss how Iago's jealousy sets the play's conflict and drives the actions and responses of each of the other characters. Without this, there would not be much of a play. Iago seems to know exactly which "buttons" to push to make each character do what he wants.

You could also describe how Shakespeare presents Iago's appearance and mannerisms, for these, too, are integral parts of his character. Iago is excellently deceptive, for instance. He is a wonderful actor who can hide his true motives and actions. The other characters don't suspect a thing. Iago completely takes them in.

Finally, you might think about how a faulty Iago would have made a less successful play. Speculate a little on what might have happened if Iago was not as jealous, manipulative, or deceptive as he is. If Shakespeare had not crafted him so carefully, we would have had quite a different play. Perhaps Othello would have seen through Iago's ploy and avoided the tragedy, but the message would not have been the same. And you should reflect on that message and how Iago contributes to it.