I read in many places that Eric Stoltz was substituted by Michael J Fox in the film Back to the future. And the main reason is that Eric wasn't bringing the screwball energy the film needed. Is this the only reason? Something else happened?
Summary of the videos - he was a good actor but they wanted something more comedic. After several weeks of filming, in consultation with producers and others, Zemekis finally decided he wasn't getting the performance he desired for this particular role and film. Lloyd himself had no idea the change was going to / needed to happen
Director Zemekis and Producer Speilberg on the topic:
Christopher Lloyd on the topic: