Gavin & Stacey star Laura Aikman
The 39-year-old actress shared a video on her Instagram showing her family screaming in shock as her character Sonia appeared in the episode.
Sonia, the ex-girlfriend of James Corden’s character Neil “Smithy” Smith, appears in the final Christmas special in a crucial plot twist.
One member of Aikman’s family can be heard shouting “press pause” while another tells her “you never told me”.
“We never told anyone,” Aikman replies.
Sharing the video on Instagram she wrote “the moment my family realise Sonia is ruining Christmas again” and captioned it: “I take an NDA very seriously.”
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She also shared a photo of a cast board of all the Gavin & Stacey characters, with a space missing where her picture would have been.
Aikman joined Gavin & Stacey as Sonia in the last Christmas Day episode in 2019, when Smithy brought his girlfriend to meet his family and friends.
But she did not get on with the group and left before Smithy could propose to her as he had planned.
Vanessa “Nessa” Jenkins, played by Ruth Jones, later got down on one knee and confessed her love for Smithy, but before he could respond to her proposal the episode ended on a cliffhanger.
Fans have waited five years to find out his answer, with the 2024 Christmas Day episode opening with the family of Stacey Shipman, played by Joanna Page, and her husband Gavin, portrayed by Mathew Horne, preparing for a wedding.
The BBC said the episode secured the highest overnight Christmas Day ratings since 2008.
The 90-minute episode drew an average audience of 12.3 million, according to overnight figures, surpassing the show’s 2019 Christmas special by more than half-a-million viewers.