Personally, I don't think the Joker was a performance. The Joker is the real Jack Nicholson.
The performance is how Jack Nicholson behaves while in the public eye.
This a quote from a semi famous model. It is from page 365 in the book "Model" by Michael Gross.
"And then we walked down Avenue George V, and who do I see coming out of the Hotel George V? Jack Nicholson. I've got to do something. So I say 'Jack, what are you doing in Paris?' I've never met Jack Nicholson in my life. Lisa's from Australia. She doesn't know who he is. So he looks me up and down, he looks Lisa up and down, and he says, 'Uh, ladies, I'm going to a little party if you'd like to join me.' "
"So we go around the corner to a little apartment, he rings the doorbell, and who answers the door but Roman Polanski. Oh, shit. We walk into a room full of fourteen-year-old blondes. We're too old, and we're looking at each other like, we've got to get out of here. Someone brings out a joint. Lisa doesn't smoke pot. She's a mom. But I smoke. I love to smoke. And all of a sudden I have to lie down. I feel really ill. I come out of it in two minutes. But there are other girls passed out, you know? Very nasty. We're out of here...."
