On Shakespearean Insanity

What is it about the transition from sanity to insanity in Shakespeare's greatest dramas—the Scottish play, Othello, Merchant of Venice, King Lear, e.g.—that we find so fascinating? Certainly, those transitions are among the draws, for the audience, for the player: is it believable?

How can one play Othello, for example, and make the audience believe that the great general, in command of armies, the public, accolades, and himself, descends into such insanity that he destroys that which he most loves?

I look forward to seeing what OSF does with the tragedy of the Moor.