In the film James takes on the role of Welsh mobile-phone salesman-turned-opera singer Paul, who first captured the nation's attention in 2007 when he auditioned for the famous talent show.
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James as Paul during filming in Wales
Directed by David Frankel, the man behind The Devil Wears Prada, One Chance stars Julie Walters and Colm Meaney as Paul's mother and father, Yvonne and Roland Potts.
Mackenzie Crook plays Paul's best friend and Carphone Warehouse co-worker and Alexandra Roach stars as Paul's girlfriend, Julie-Ann Cooper, who he met online in 2001.
The film charts Paul's life starting with his experiences as a bullied schoolboy, meeting his wife Julie-Ann for the first time at Swansea railway station and the fateful moment he applied for Britain's Got Talent.
Paul promoting the film
Rumours have also been circulating the Simon Cowell, who signed Paul to his record label Syco, will play himself in the film.
Speaking during filming James revealed that music mogul Simon had showed his support for the biopic.
"I haven't met him or talked to him, but one of our producers is in touch with him a lot," said James. "He's been very supportive from afar.
"He loves the story – it's just a testament to the power of his shows."
Over the past year James and the rest of the cast have been spotted filming in Wales and Venice, where Paul attended an opera master class before he rose to fame.