Answer: Give Actors Vacation Time

It would be easy to assume that kryptonite, the radioactive ore from the planet Krypton, was introduced into the Superman mythos specifically to give the Man of Steel a weakness. After all, without kryptonite Superman is unstoppable, and a completely unstoppable superhero, no matter how benevolent he might be, definitely makes for boring plots.

The primary motive for the introduction of kryptonite wasn’t careful plot consideration, however, although it certainly ended up being a very important component of Superman storylines big and small over the years. Kryptonite was introduced in 1943, not in the comic book but in the Superman radio show specifically to provide a mechanism which would render Superman unable to speak and, in the process, give voice actor Bud Collyer a chance to sneak in some vacation time. While Collyer was on holiday, all of Superman’s speaking parts, limited to incoherent mumbling and moans of pain, were filled in by other voice actors who could easily perform the task without having to actually sound like Collyer.

Kryptonite didn’t enter into the comic canon until 1949 when it was introduced in printed form via Superman #61 (more than a decade after Superman’s first print appearance in Action Comics #1).