In the scene where David McCallum appears to be playing the first prelude from Bach's Well Tempered Clavier, the camera pans down from his face to his fingers and it seems he is actually playing the piano himself. This is highly likely since McCallum, like the rest of his family was a trained musician who went to the Royal Academy of Music and originally intended a career as an oboist.
To fill extra time in a specific scene, Director James Goldstone added a musical sequence involving the central Character Gwyllim. The significance was not to build new sets or add extra characters but to be cost effective. Gwyllim is seen actually playing Bach's preludes on the piano as actor David McCallum practiced the piece prior to the scene hitting the right keys but the actual music was dubbed over.
David McCallum (Gwyllim Griffiths) later played Joshua Hayward in Feasibility Study (1997). Along with Leonard Nimoy, Cliff Robertson, Barbara Rush (who appeared with him in The Forms of Things Unknown (1964)) and Peter Breck, he is one of only five actors to appear in both The Outer Limits (1963) and The Outer Limits (1995).
When Gwyllim explains the things that impedes man's development, in the original script he lists "morality." For fear of offending viewers it was changed to "immorality."
Professor Mathers' chimpanzee Darwin was played by stuntman Janos Prohaska. Three years earlier, Prohaska had also played a chimp named Darwin, in Wagon Show (1960).