William Russell was originally scripted to reprise his role as Ian Chesterton, but was forced to drop out due to scheduling conflicts. This resulted in the choice to instead bring back the Brigadier.
Ian Marter was considered to reprise his role as Harry Sullivan for this serial, but he was unavailable.
In the script, it is suggested that part of the reason the Doctor is so accepting of Turlough is because he still misses Adric. This motive was eventually dropped, however.