The Mysterons themselves are never seen on screen. They broadcast their threats by radio, often disguising their intentions with word play – such as stating that they will "kill time" when they intend to kill a major Spanish official whose surname is Tiempo (Spanish for "time") in the episode "Operation Time". The original script for the first episode described the voice of the Mysterons as "commanding" and "hypnotic", also specifying that it would be created "by the artist inhaling helium and the subsequent recording being played back at a lower speed, together with a repetitive echo treatment". The aliens are represented visually by twin rings of green light that they cast onto the scenes of murder and destruction from which their facsimiles emerge. The pilot script described these rings as "two fiery green circles" suggestive of eyes. Gerry Anderson said that he came up with this device after seeing a TV advertisement for wool that featured the Woolmark logo being projected over the body of a woman. During filming, the ring effect was produced by panning a transparency across the set using a slide projector.