Sean Penn has said that if Christopher Nolan ever considered him for the role of the Joker in The Dark Knight, it was never mentioned to him.
Nolan surprised everyone by casting Heath Ledger instead, which was met with some fan backlash at the time.
Joaquin Phoenix is currently considered the favorite for the role, though in a recent interview he denied knowing of its existence.
It’s hard to imagine another actor playing that role now, but for years it’s been suggested Sean Penn was actually Nolan’s first choice, but he turned it down.