Reason why Gavin and Stacey star was banned from cast party and forced to sleep in separate hotel

Warning: This article contains spoilers for Gavin and Stacey: The Finale

There's a pretty good reason why one Gavin and Stacey star was banned from the cast party and forced to sleep in separate hotel while filming the long-awaited finale special.

BBC viewers couldn't wait to finally watch the Christmas finale episode which saw Smithy (James Corden) engaged to Sonia (Laura Aikman) yet - at long last - finally tie the knot with Nessa (Ruth Jones).

After what felt like a lifetime of waiting for the pair to finally say 'I do' (or 'tidy') to one another, fans of the TV show were keen on finding out how producers were able to keep Sonia's involvement in the finale a secret for so long.

Writers actually went to rather extreme lengths to achieve this level of mystery, making the decision to ban actress Laura from any cast reunions during the build-up to the festive finale.

TV critic Scott Bryan has since spilt the beans on how the gang kept everything hush hush for so long.

"Sonia stayed in different hotels than the crew to protect the plot," he tweeted after the episode aired on Christmas Day (25 December).

"She didn’t even tell her parents she was in it, who are tuning in to the finale tonight alongside her."

Gavin and Stacey viewers were not so chuffed about Sonia's return as Smithy's fiancée (BBC)

And after some social media users simply couldn't believe that Laura had even been able to keep the secret from her family, the actress shared a video on Instagram of her parents watching the ep, their jaws dropping when it was revealed that she'd be marrying Smithy.

"The moment my family realise Sonia is ruining Christmas again…", she captioned the hilarious video, in which her mother could be heard yelling, 'Oh my God?! What? You never told us!', before Laura replies: "We never told anyone!"

She captioned the clip: "I take an NDA very seriously."

Smithy and Nessa sealed the deal with a handshake (BBC)

The episode's director Christine Gernon has since also opened up on the grand plan, writing online: "@Lauraikman spent the whole G&S shoot being hidden away.

"Staying in a different hotel, not allowed to come to wrap or screening parties so no-one knew Sonia was back.

"Such a great reveal. And she’s amazing as both actor and person."

And as we say, though fans were horrified by Sonia's return, they were amazed by the level of secrecy they'd been able to maintain.

"I heard the whole UK gasp!" one wrote on Twitter, with another penning: "Gotta deal with the insufferable Sonia on Gavin and Stacey AGAIN?!"

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