James Brown in his early years.

At first Oscar-winning actress Octavia Spencer thought she was seeing early concert footage of James Brown but then was told it was a screen test by Chadwick Boseman to play the ‘Godfather of Soul’ in the new biopic Get On Up.

From the gliding, intricate footwork to the hunched shoulders and fluid movements, Boseman had nailed Brown’s singular mannerisms and raspy voice.

“It was just him from the back, and then from the side, and it was all in silhouette with the hair. I could not believe it, and he had only been learning the dance for three days,” said Spencer, 44, who plays Brown’s no-nonsense, knife-wielding Aunt Honey in the film, which opens in US theatres today.

Spencer, the 2012 Academy Award-winner for The Help, said Boseman immersed himself in the role of the three-time Grammy winner, who died on Christmas Day in 2006 at the age of 73.

Get On Up follows the man and his music in flashbacks from his impoverished early years in a shack in rural South Carolina, to living in a brothel run by Aunt Honey and singing in a local church, to his early career performing with the Famous Flames and later international stardom.

Behind the glitzy costumes, pompadours and showman’s bravura, the film, produced by Mick Jagger and Brian Grazer, shows Brown as an industrious teen scrounging a living, a troubled, perfectionist musician, an egotistical performer and a shrewd businessman.

Bozeman, 32, is no stranger to playing iconic figures. In a break-out role, he portrayed black baseball pioneer Jackie Robinson in the 2013 biopic 42, but bringing one of the most influential figures in popular music to the big screen was a challenge.

“I was scared of it. There was no part of it that was straightforward and easy,” said Boseman, who like Brown, hails from South Carolina.

Mastering the dance moves and getting past the caricatures of Brown, famously parodied by Eddie Murphy on the comedy sketch show Saturday Night Live, posed the biggest challenges.

Boseman worked with a choreographer, spent time in the South to get into the part and spoke to Brown’s family to capture the essence of the man.

For Jagger, finding an actor who could dance and depict Brown’s charisma in the film, which was directed by Tate Taylor (The Help) was important. Jagger and the Rolling Stones played on the same bill with Brown in 1964 at a live concert in California. As a performer influenced by him, Jagger wanted to find just the right actor to play Brown.

“For mu-sicians and for listeners and dancers alike, I think he has made this huge contribution, this lasting contribution which goes on. I think this movie does, hopefully, that legacy justice,” said Jagger.

Judging from early reviews, the film-makers got it right with Boseman.

“It’s that rare musician’s biography with a deep feel for the music. And in Chadwick Boseman, it has a galvanic core, a performance that transcends impersonation and reverberates long after the screen goes dark,” said the Hollywood Reporter.