In 1973, Doctor Who producer Barry Letts gave the role of Sarah Jane to April Walker, who has starred in Fawlty Towers, The Two Ronnies and Yes, Minister.
However during rehearsals for The Time Warrior, Letts thought there was a lack of chemistry between Walker and co-star Jon Pertwee and re-cast the role to Elisabeth Sladen.
According to Elisabeth Sladen: The Autobiography, Pertwee did not get along with Walker.
The news was revealed on the DVD release of Sarah Jane's second story, Invasion of the Dinosaurs.
(Source: Radio Times)