Letts had intended to cast an older actor as the Fourth Doctor, to hark back to Hartnell's portrayal in the 1960s, but after a long search he eventually selected Tom Baker, who was suggested to him by the incoming Head of Serials, Bill Slater.
Letts zamierzał wybrać na czwartego Doktora starszego aktora, by powrócić do wizerunku Hartnella z lat 60., ale po długich poszukiwaniach ostatecznie wybrał Toma Bakera, którego zaproponował mu powracający kierownik Działu ds. Seriali, Bill Slater.