(Image via David Shankbone)
It's hard to imagine anyone else besides Jack Nicholson as the Joker in Tim Burton's Batman, but as it turns out, he wasn't actually Burton's first choice. Instead, John Lithgow was his first choice. Lithgow turned down the role (as did Jack Nicholson at first), but now he regrets it, saying he had no idea the role would be so big. I'm sure it also didn't hurt that thanks to a profit-sharing agreement, Nicholson earned an amazing $50 million from the film.
You can read what John Lithgow had to say about the whole thing on Vulture.