Imagine the phone call.

"This is Fred down in makeup for Batman. We're doing Cesar Romero, right, and he says he won't shave his mustache."

"What?"

"He won't shave it off."

"He's the Joker."

"Yep."

"His face, it's all white."

"Yep."

"So, what do we do?"

"Well, we could just put the white on there."

"Over the mustache?"

"Right. Or we could create an appliance to cover his whole upper lip."

"A fake lip?"

"Uh-huh."

"Wouldn't he talk funny?"

"He kinda talks funny already."

"Well, Hell. We're overbudget anyway. Just cover it up."