April Walker

 was the original actor cast and contracted for Sarah Jane Smith. After an arduous audition process, Barry Letts formally awarded her the role on 14 March 1973. However, he had not involved Jon Pertwee in the process and realised, too late, that Pertwee didn't approve of the choice. Letts therefore quickly replaced her with Elisabeth Sladen, someone who had not been involved in the auditions that produced Walker. (INFO: Invasion of the Dinosaurs)

Despite the fact that she had been effectively fired, Walker was paid in full for the whole of season 11 as her contract demanded. Her identity was a closely-guarded secret and something neither Barry Letts nor Elisabeth Sladen revealed in their lifetimes. It was only uncovered by info text writer, David Brunt who happened across an order for her payment in the course of research on the Invasion of the Dinosaurs DVD.