"It was acting!
Errol Flynn in The Rocketeer. (Well, officially Neville Sinclair.)
And Tiny Ron as Rondo Hatton made the best hench-villain too.
The competition for worst is ferocious, but for sheer stupidity Sebastian Shaw in X-Men First Class has got to be considered. Killing a kid's mother is the foundation for a partnership in conquering the world? His superpowers disappear so that he can be killed by a slow moving coin? Kevin Bacon is so compelling that you almost buy it, but no, not quite.