The computer was voiced by John Leeson, who previously played the Doctor's robot dog companion K-9. Leeson was asked to make his voice sound like Davros', to trick viewers into thinking the computer was Davros, and watched past episodes for reference. Michael Sheard was chosen to play the headmaster as he would be familiar to children. Sheard had to be released from his work on "Grange Hill" to participate; Peter Tilbury was briefly considered for the role if Sheard could not make it. Sheard had previously appeared in "The Ark" (1966), "The Mind of Evil" (1971), "Pyramids of Mars" (1975), "The Invisible Enemy" (1977), and "Castrovalva" (1982). Peter Halliday, who played the blind Vicar, had also appeared in various "Doctor Who" stories. Stratford Johns, who had previously appeared in "Four to Doomsday", was originally considered for Ratcliffe. Mark McGann, the brother of Eighth Doctor actor Paul McGann, was originally considered for the role of Mike Smith. Pamela Salem had roles in two Fourth Doctor serials, as one of the Xoanon voices in "The Face of Evil", and as Toos in "The Robots of Death" (1977). Simon Williams, Karen Gledhill and Pamela Salem reprised their roles in this serial in an audio spin-off series for Big Finish titled "Counter-Measures", which details the adventures of the group after this story.