Even with all the money and behind-the-scenes talent in the world, the majority of Hollywood's tentpole movies are sold and dependent upon the merits of their casts above all else.
Imagine if Clive Owen had really been cast as Tony Stark instead of Robert Downey Jr., or Hugh Jackman never accepted the Wolverine role. Would these parts have been as quintessentially iconic in other hands?
What state would the Marvel Cinematic Universe be in without Downey Jr.'s quick-fire wit? And if Fox hadn't plucked a relatively unknown Aussie stage actor to play Logan, would people be openly weeping about the end of his latest movie?
Casting actors in unexpected roles, often against-type, is a major risk, but as the history of cinema has proven many times over, even performers as generally-reviled as Adam Sandler can prove surprising with the right material and collaborator.
These 18 roles, from oddball cameos to nervously-anticipated recastings of beloved characters and even a reprise from beyond the grave, have rightly raised more than a few eyebrows from film fans. As ever, though, time will reveal whether or not they were worth it...