Christopher Reeve passed on a planned Superman cameo in 1984 spin-off Supergirl. Superman 1978 marked the big screen debut of the titular superhero, and it became Reeve's defining role, with the actor perfectly embodying what made the superhero so special. Reeve was always honest about the shortcomings of his later Superman films. While Superman II was another success, the fortunes of the saga took a downturn with 3 and 4. Superman III brought in comedy legend Richard Pryor to play a hacker but became more of a wacky comedy as opposed to a Superman film. Superman IV was a box-office disaster that doomed the series and marked Reeve's Superman appearance.
Sandwiched between III and IV was Supergirl. This cast Helen Slater as Superman's cousin who comes to Earth to retrieve a Kryptonian artifact, and she comes to blows with Faye Dunaway's witch. Supergirl was originally to appear in a more ambitious version of Superman III, where she would have been Superman's love interest, not his cousin. Reeve himself was the one to suggest Jeannot Szwarc direct Supergirl after the two had worked together on romantic fantasy Somewhere in Time, but three months before production began, he told the director he "didn't feel right" (via documentary You Will Believe: The Cinematic Saga of Superman) about the spin-off and exited.
Superman Was Supposed To Be Supergirl's Mentor (Would It Have Worked?)
In the final movie, it's mentioned that Superman has left Earth on a "peace-seeking mission" to another galaxy, explaining his absence. The original plan for Supergirl was that Reeve's superhero would mentor his cousin and show her how to use her powers before he needed to be saved after being cursed by a spell. A couple of factors likely caused Reeve to back away from the Supergirl movie. Following the release of Superman III, Reeve admitted to being unhappy with how the sequel had turned out and felt its comedy focus hurt the final product.
In the aftermath of its release, Reeve stated in numerous interviews he was done playing Superman. While Supergirl went into production a few months before Superman III's release, he was probably sensing how it would be received. It's also possible he was concerned about how closely linked to the role he was in the public's consciousness and felt if he didn't branch out to other parts, he would be forever typecast. It was only the lure of a big cheque and creative involvement that got him to commit to Superman IV which - ironically - received much worse reviews and failed to recoup its budget.
Reeve Would Have Considered Superman V
Of course, even if Reeve - who later passed on a Hannibal role - had only appeared for ten minutes in Supergirl, his scenes likely would have been considered a high point. This would have made the links between the films much tighter too because his absence is keenly felt in the final product. While Reeve's was burned by Superman IV's reception, he stated to Dixie Trek convention (via YouTube) in 1994 he was open to a return - if it had a good story and the budget to make it a big event. Tragically, Reeve would be left paralyzed following a horse-riding accident the next year, and such a return became impossible.