Edward Norton in The Avengers

Edward Norton is known for playing the titular role in The Incredible Hulk (2008), so logically, he was expected to reprise the role in the Avengers films. But when negotiations between him and Marvel Studios broke down, Mark Ruffalo was brought in as his replacement. It's worth noting that Mark was also considered for the role in The Incredible Hulk before Edward was ultimately chosen. A fair trade if you ask us!