How it could have been: nearly 40 years later, the secret of who was originally cast as Doctor Who's Sarah Jane Smith has been revealed. It was April Walker, best known for myriad guest roles in Fawlty Towers, The Two Ronnies and Yes, Minister.
Pictured above left is Jon Pertwee in the 1973 story The Time Warrior, which introduced Sarah Jane Smith. Then, above right, is April Walker in a Two Ronnies sketch, The Attractive Barmaid, from the same year.
Doctor Who producer Barry Letts cast Walker when Katy Manning left the role of Jo Grant, the Doctor's previous companion, and she worked in rehearsals for The Time Warrior. But allegedly the pairing of Pertwee's Doctor and Walker's Sarah Jane didn't work: she was a tall and more obviously strong character, along the lines of Pertwee's first companion, Liz Shaw (Caroline John).
It's believed that Pertwee was unhappy with the decision but it would ultimately have been Letts who recast the role. Walker was reportedly paid for both The Time Warrior and the rest of that series.
Changes during production are not common but they do happen: the most famous is perhaps the replacement of Eric Stoltz with Michael J Fox for Back to the Future. Unusually, that was quickly revealed, even if footage of his filming has been harder to find. But for BBC drama fans, the remaining question is over Dr Rose Marie in A Very Peculiar Practice: Barbara Flynn knows who she replaced during filming but still won't say.
Lis Sladen always refused to reveal April Walker's name, too. But the news has been divulged now on a DVD release of Invasion of the Dinosaurs, the second story with Sarah Jane.