Danish bodybuilder Sven-Ole Thorsen played the role of the gladiator Tigris of Gaul in the 2000 action film Gladiator. But the original plan was to cast Lou Ferrigno, the actor most famous for playing the titular role in The Incredible Hulk.
Thorsen convinced directors that he was the right man for the job, so he took the Hulk's job away from him, hopefully not making him angry in the process.
That's 1 of 10 sudden casting changes made while a film was in production, sometimes creating enormous editing challenges. You can watch more in this video by Screen Rant.
-via Geek Tyrant