The Replacement's original ending would have been very different

The drama, which saw McClure's Paula driven to murder, was at one stage set to feature Paula's deceased daughter Caris alive and revealed as the person responsible for pushing Kay through the library skylight.

"In a very, very, very early version of the story, we flirted with Paula’s daughter being the one who did the deed," Ahearne tells RadioTimes.com, explaining that certain fan theories about the show matched actual plot lines the team considered and later rejected.

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"We discussed that a bit because we thought that could be fun. Then the explanation for Paula’s behaviour would be she’s trying to protect her child and she’s fighting for her child. But the more we thought about it we thought, well, really the drama is between these two women so it’s really got to be Paula at the end because it’s about the two of them. And also Ellen (Morven Christie) is the heroine so I want Ellen to be right at the end of the day."

In fact, the drama's concluding part saw Paula admit to Ellen that it had been her who killed architect boss Kay at the end of episode one after she began digging into Caris's death. "She wouldn't shut up about Caris," she said. "I just wanted her to shut up."

Ahearne has no regrets about changing the direction of the tale because he believes her daughter's demise provided Paula with a "much more powerful motive" for taking her frustrations out on Ellen throughout the series.

"The reason why Paula is being driven crazy by Ellen’s behaviour is because she comes into work every day and she sees this woman, in her eyes, who’s not putting the daughter at the centre of her life and doesn’t understand that to lose your daughter is the worst thing in the world and as that escalates, I think that’s a better motivation", he says.

That idea that Paula's daughter would be the one to kill the architect wasn't the only direction they considered either.

"Another fan theory we had toyed with and rejected was Paula being an ex-patient of Ian's," Ahearne adds.

Well done to the fans who put those theories forward – you should have a stab at becoming a BBC drama writer some day.

https://youtu.be/7QvatznQX9g