Back to the Future celebrated its 25th anniversary this past summer. In the near-instant classic, Marty McFly, a resident of 1985, uses a time machine/Delorean created by idiosynchratic inventor Doc Brown, and travels back in time three decades, meeting his pre-courtship parents, making a mess of things, and in the end, fixing it (and making his life in 1985 better). The movie yielded two sequels, each of which were successful in their own right. But for the cast of the first movie, the rest of the triology was not nearly as successful. Two actors from the first movie did not reprise their roles. Claudia Wells, who played Marty’s girlfriend, Jennifer Parker, turned down the opportunity to return to the Future franchise, citing her mother’s ailing health, and was replaced by Elizabeth Shue. Crispin Glover, who played Marty’s father, also did not return, claiming that the wage offered him was less than half of the other returning cast members.
The producers replaced Glover with some magic and, probably, creative scriptwriting.
In Back to the Future Part II, the character of George makes few appearances. Marty, on the urging of Doc, travels to 2015 to save his future self from a series of poor decisions. While in the future, a miscue on Marty’s behalf results in disaster: nemesis Biff Tannen finds out about the time machine, travels back to 1955 to coach his high school self, and sets off a series of events which changes history. Upon Marty’s return to 1985, reality has taken a dystopic turn: Biff Tannen is the area’s richest man (and not afraid to show it!) while Marty’s father George is dead. Marty travels back to 1955 to remedy the situation.
In the future, we see very little of George (and only upside down). In the dystopic present, he’s dead. And in the past, we see the same George we saw in the first movie. And in filming, the producers took advantage of these facts. George McFly was portrayed by two actors in Back to the Future II: Crispin Glover and Jeffrey Weissman. Glover’s segments were, in fact, repurposed footage from the first movie. Weissman was fitted with a false chin, cheekbones, etc., while using various obfuscation effects (wearing sunglasses, being filmed from behind, or, yes, upside down, etc.) in order to look like the George McFly character as performed by Glover.
Glover successfully sued the producers, claiming they were trading on his likeness without his authorization. Since then, the Screen Actors Guild has inserted a prohibition on conduct used by the Back to the Future producers into its collective bargaining agreement.
Bonus fact: The opening scene of Back to the Future II involves some footage from the first movie. Because Elisabeth Shue replaced Claudia Wells, some parts of these scenes had to be reshot; a comparison can be seen here.