What Casting Choice Sounded Terrible At First, But Turned Out Great?

Sometimes, a casting announcement will make our eyes roll, but it's a real treat when an actor we couldn't see in a particular part blows us away with a stellar performance. Which actor surprised you the most in a role you didn't think they could pull off?

Naturally, Heath Ledger's Joker springs to mind, but there are plenty of actors who have proved a far better match to a character than we'd expected. Tell us which actor in which role surprised you most.