September 29, 2015 at 06:39 PM EDT

Eric Stoltz’s casting as Marty McFly — and then his being replaced by Michael J. Fox after filming began — has become an infamous aspect of the Back to the Future legacy and one of Hollywood’s most obsessed-over what-ifs. In an exclusive clip from the upcoming Back to the Future documentary, Back in Time, director Robert Zemeckis isn’t shy about what led to the switch.

Watch the clip above to hear Zemeckis’ thoughts on choosing Stoltz after his original choice, Fox, was unavailable due to his TV commitment with Family Ties. Lea Thompson also speaks to Stoltz’s casting, as the two previously co-starred together in the Cameron Crowe-penned The Wild Life.

“Eric is a really good actor, and I made the decision to put him in the movie, but it turned out that his instincts and the type of the comedy [of] the film we were doing weren’t really gelling,” Zemeckis says.

Back in Time will release in select theaters and on VOD services on Future Day, a.k.a. Oct. 21, and is already available for pre-order on iTunes.

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