Look for Iago

In her contribution to Bringing the Devil to His Knees, Susan Neville reports receiving this pithy bit of advice from Kurt Vonnegut:

…he asked how my novel was going.
         It's a mess, I said, truly awful, and I tried to think of some way to change the subject.
         I can tell you what's wrong with it without seeing it, he said. You're missing Iago.
         Iago?
         The character that bounces all the other characters around. I wasted twenty years figuring that out. Look for Iago.

From Susan Neville, "Where's Iago?" in Charles Baxter & Peter Turchi, eds., Bringing the Devil to His Knees: The Craft of Fiction and the Writing Life (2001).