One of the most famous instances of replacement casting has to be the Back to the Future Eric Stoltz/Michael J. Fox switcheroo. If you're not familiar...well, you're about to be. Long story short, BttF director Robert Zemeckis made a last minute change to the casting of the time travel classic...a couple weeks into filming.

In the clip, Zemeckis explains how it went down and for the first time, you'll get a glimpse of Stoltz acting in some of the movie's classic scenes. A true gift of Blu-ray box sets!

Don't forget to go back in time to our treasure trove of Sci-Fi: The World of Tomorrow and check out Back to the Future 25th Anniversary Trilogy when it hits Blu-ray and DVD October 26th.