I've had this mental image of the Joker in my head for years, as part of a vague set of ideas for a reboot of the character. In my mind, Bruce Wayne is touring Arkham (pretending that it's a mad whim, the sort of morbid curiosity of the idle and politically-connected rich) and they show him the cell. In it, there's a man with chalk-white flesh and bright green hair, laughing and laughing and laughing and laughing...
"Who's he?" Bruce asks.
"We don't know his name. We found him five years ago, sitting just like this in a fast-food restaurant, dressed up like their mascot."
"Why didn't you clean him up?"
"We tried. That's not make-up. His actual skin is like that. Hair, too. Toxicology shows it to be some sort of chemical dye, the same sort of thing we found in the victims."
"Everyone else in the building was dead. We think he was handing out poisoned food, pretending it was free samples. We may never know for sure."
"Haven't you asked him?"
"He hasn't stopped laughing. He doesn't talk, he doesn't sleep, we acutally have to force-feed him. He just keeps laughing like that. We can't put any prisoners in this row, they go nuts." Pause. "More nuts."
Bruce looks at the man in the cell, trying to see through those eyes for a moment to understand the madness within...and then shakes himself free of it. The tour moves on.
That'd actually be the last you see of the Joker for a while. Maybe twenty issues, maybe twenty-five. Batman goes on, fights other bad guys...and then at the end of a storyline, after the main action is done, the scene cuts to Arkham. The panels form a long "tracking shot" down the hallways, following a trail of laughter sound effects to its source: The man in the cell. He's laughing, and laughing, and laughing, and laughing...
And slowly, the laughs become giggles. The giggles become chuckles. The chuckles subside into that last little, "heh, heh, heh...whoo," that comes at the end of a long fit of giggles. And the Joker wipes a tear of laughter from his eye.
"Well," he says, "that was fun. I wonder what I should do next?"