Trivia

The voice of the Mysterons, and hence that of Captain Black as a Mysteron agent, was produced by having Donald Gray read his lines as its provider at normal speed whilst the tape recorder was accelerated; when the tape recording was played back at normal speed, the sound of Gray's voice became deeper and slower than his own, and hence more ominous and menacing.

Real human hair was used for the marionette heads. Photographs of actual human eyes were used to increase the realism of the marionettes. After producing several series using cartoon-like marionettes with large heads (required to contain the electronics that operate the puppets' eyes and mouth mechanisms), Gerry Anderson was able to miniaturize the mechanisms further starting with this series, allowing more realistic-looking puppets to be used.

According to Gerry Anderson on the documentary for the 2001 DVD release, The Mysterons were written as an invisible enemy in case it was proven that there was no life on Mars, he could say there is that you just can't see it.

According to background information about some of the characters produced by Century 21 for its publications, "Captain Scarlet" takes place a few years after the events of Stingray (1964), with some SPECTRUM personnel actually veterans of the earlier show's WASP organization. None of this was mentioned on screen, however.

For the most part, the puppet characters' faces were modeled after the voice actor portraying them.