On screen, they’re Earth’s mightiest heroes. But in Hollywood, they were has-beens and never-weres. That is, until Marvel got a hold of them.

“Avengers: Age of Ultron” kicks off the summer movie season May 1 with a whirlwind of superheroes joining forces to once again save the world and shatter box-office records. The director and several key stars in the ensemble cast either kick-started their film careers with Marvel Studios (Chris Hemsworth as Thor, Chris Evans as Captain America, director Joss Whedon) or staged a dramatic...