I did a lot of work on my antagonist a few months ago, mostly in the form of notes. First I figured out who he was and how he fit in a general way into the story. I did this by brainstorming names until I found one that fit. Then I wrote the name at the top of a blank page and asked “Who are you?”
Later, after I had a little bit of info, I was able to give him motivation and a concrete goal. Then I twisted it to put someone important to my plot at risk. That’s as far as I got, except I sort of know how he gets caught and one or two things that have to happen before that.
What I didn’t know was how to incorporate him more completely with my characters. They’ve met, of course. I put him in the first chapter and he’s made appearances a few times since. But they are easily missed because I don’t want to telegraph “This is the murderer!”
So yesterday I did the McKee exercise and there in the final opposition I found out how he was going to ingratiate himself further into the story. It’s perfect. It strengthens the betrayal, makes everything worse. And I would not have figured it out if I had not done that exercise.