“The music industry prizes authenticity above everything,” says James McAvoy. “It’s an art form about turning your own story, your own heartbreak, into the show, isn’t it?” The 46-year-old sighs and shakes his head. “But what if your narrative isn’t one the music industry thinks it can sell? What then?”

This is the dilemma faced by the heroes of McAvoy’s rollercoaster ride of a directorial debut, California Schemin’. The film is based on the true story of Dundee-born hip-hop artists Gavin Bain and Billy Boyd (AKA Silibil N’ Brains), whose Scottish accents caused them to be mocked as “the rapping Proclaimers” when auditioning for a record deal in the early 2000s.

Authenticity rejected, the 21-year-olds (who were living on benefits and had never been to America) reinvented themselves as brattish Californian jackasses, spun absurd lies about gangsta rap contacts for the London suits and were almost immediately signed by Sony. They were handed a £75,000 retainer, opened for iconic rap collective D12, partied with Madonna and Green Day, appeared on MTV and recorded three albums’ worth of material before their ruse was rumbled.

Boyd has since said the stress and paranoia generated by maintaining the fiction began to eat away at them. They were locked into legal contracts that made them scared to expose the whole prank as planned and began to suspect the characters they’d created were more likeable, more bankable than they were.