James Corden has confessed that all his singing vocals are dubbed in new movie One Chance in which he plays Britain's Got Talent winner Paul Potts.
Although he is a competent singer and about to begin filming Hollywood musical The Woods, Corden admitted his opera is not up to scratch.
So Potts's own voice was dubbed over that of the Gavin And Stacey star.
Corden was at the Toronto Film festival for the world premiere of One Chance, where he was joined by Taylor Swift - who composed a song for the movie - along with Alexandra Roach, who plays his on-screen wife, and Potts himself.
He revealed: "It was very, very straightforward - I would pretend to be singing opera, and then Paul would go into the studio and loop to my mouth. I hope I haven't given it away too much, but that's basically what happened."
The actor also admitted that he found it less pressure to play Potts because the film ends at the moment he walks into his first Britain's Got Talent audition, the point at which the nation came to know him.
Swift revealed that she signed up to the project because she was so moved by the story of the mobile phone shop worker from Wales who shocked everyone with his incredible voice.
She said: "It's amazing to be a part of this project. It's a movie that really moved me in a way that I haven't been moved in a long time, and to to have just a small part in telling the story... I wrote the end-credit song so it's the last thing you see of the story and I'm really, really honoured."
The Iron Lady star Roach recalled watching Potts win Britain's Got Talent back in 2007.
She revealed: "I was in hospital at the time and I remember me and three old ladies gathered around the communal television in our ward, and we watched him sing.
"And because he was from Swansea and I was in a South Wales hospital then, we were all crying and weeping and really rooting for him. And it's so bizarre now to think that I'm sharing a red carpet with him and I'm playing his wife. It's mad."