You need only look so far as this bravely archived article about Heath Ledger being cast over Paul Bettany to see how many people thought that Bettany was a better choice. He was widely believed at the time to be the front-runner to win the role, so when Ledger took the part, there was a lot of shock and disappointment among comic book fans. Would It Have Worked?: Certainly. Bettany may lack the awards plaudits of Ledger, but he's been a consistently creepy presence in a lot of not-so-great films: The Da Vinci Code, Legion and Priest all benefited hugely from his stature even if the films pretty much sucked. His unhinged potential would have made him a great Joker for sure, though at the same time, would it have denied him from also working as Jarvis in the Marvel Cinematic Universe?