What comedy legend was almost a big-screen Batman villain twice?

The answer is: Robin Williams. Robin Williams was initially offered the role of Joker in Tim Burton’s 1989 Batman, but lost out on the part when Jack Nicholson showed interest. The studio decided to go for the bigger star. Williams would also be offered the role of The Riddler in Joel Schumacher’s Batman Forever, but ultimately decided to leave the role due to worries about Shumacher's working style. Jim Carrey would then swoop in and land the gig.Source