Michael J. Fox Brought In After 7 Weeks Of Filming
The video below is a comparison of Eric Stoltz who was the original actor who played Marty McFly and Michael J. Fox who would later replace Stotlz.
After filming for several weeks, Director Robert Zemeckis was unsatisfied with the films progress, because of Eric Stoltz’s performance. They fired Stoltz, and brought in Michael J. Fox who worked at Family Ties during the day, and worked on Back to the Future at night.