Eric Stoltz was the original Marty McFly in Back to the Future. From all accounts, his tone was different from the movie's intent, and he was playing it more dramatic and intense (for example, he said that he thought it was sad in the movie how no one remembered anything, which was not the tone intent of the two Bobs). Tom Wilson, who played Biff, said that he was a total method actor on set and made everyone call him Marty and acted like his character, and projected a lot of anger towards Tom Wilson and was very rough in fight scenes, and Wilson was going to punch him if he kept it up. He was fired five weeks into shooting (and after several million dollars of shooting) and replaced by Michael J. Fox, who had better comedic timing and sensibilities.
But what if Eric Stoltz had not been fired and he was kept on to just finish up the film?