GAVIN and Stacey superstar James Corden has been secretly performing in a quirky West End Show after the success of the hit BBC One series' Christmas special.
The Sun can reveal the actor, 46, has been staring for free in immersive experience You Me Bum Bum Train.
The hour-long walkthrough play, which is said to have triggered panic attacks in guests, takes place at a top secret location.
The venue, hidden inside a disused office block, is just metres away from unsuspecting shoppers in central London.
A source said: "James had been to see the show eight years ago when it was in London before so when he saw it was back he wanted to take in the madness himself.
"All actors in the play volunteer for free - regardless if you're an A-lister or have done little more than attended drama class at school.
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"The whole thing is shrouded in secrecy and no one is meant to talk about what goes on but some of the biggest names in the world have taken part.
"Guests were stunned to see James taking part but obviously, as per the rules, couldn't react."
Gavin and Stacey's Christmas Day special pulled in 19.3 million viewers.
Gavin & Stacey: The Finale broke the show’s own record from the 2019 Christmas special which reached 18.5 million viewers in 28 days.
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The 2024 special It is now the most watched scripted show of the century.
James relocated to the UK from California last year after calling time hosting The Late Late Show.
He then enjoyed a short run at London’s Old Vic in the play The Constituent from June 13 until August 10.
Founded in 2004 by Kate Bond and Morgan LLoyd, You Me Bum Bum Train, which is attended by just 77 ticket holders per show, has gained a cult-like following.
While some guests have reportedly suffered panic attacks, others have cited the experience as "life changing".
Over the years everyone from Madonna to Ian McKellen are said to have attended while earlier this week The Traitors host Claudia Winkleman risked the wrath of bosses by confirming she attended as a guest earlier this week.
Though not every famous name invited is keen to entertain the madness.
The Sun can reveal Oasis legend Liam Gallagher, 52, turned down an invite to attend one of the shows - favouring a night in with his dog at his home near the Cotswolds.