Keira Knightley and Mark Ruffalo star in Begin Again

John Carney's feel-good musical comedy drama is about starting over.

Mark Ruffalo and Keira Knightley in a scene from Begin Again. Courtesy The Weinstein Company / AP Photo

The British actress Keira Knightley, known for playing tragic heroines in period dramas, strays into new territory with her first major singing role in Begin Again, a feel-good film about the music industry and starting over.

The film was written and directed by the Irishman John Carney, whose 2006 indie musical Once nabbed the best original song Oscar for Falling Slowly. It was also adapted into a Broadway show and won eight Tony awards, including Best Musical.

Like its predecessor, Begin Again is laced with music. Much of it is sung by Knightley, along with the Maroon 5 frontman Adam Levine, who makes his film debut.

"The whole thing was out of my comfort zone. I am not a singer. I didn't know how to do that," says Knightley, a Best Actress Oscar nominee in 2006 for her role in Pride & Prejudice.

“A lot of the film was improvised and I had never done that before. I’d say that those were two major things to be grappling with,” adds the 29-year-old actress, who took voice lessons to prepare for the film.

She plays Gretta, a British songwriter and reluctant singer who arrives in New York with her boyfriend Dave, played by Levine, just as he is about to hit the big time.

Fame clouds his judgement and he strays, leaving a bereft Gretta on the verge of returning home when she is discovered by Dan, a chain-smoking, down-and-out record producer played by the Oscar-nominee Mark Ruffalo.

“Essentially, what it is about is people falling down in life and trying to pick themselves back up, whether that is romantically or whatever,” says Knightley.

Dan persuades Gretta to record her songs live on the streets of New York for a CD to revive his own non-existent career. Along the way, the two strike up an unusual ­friendship.

“I like the meditation on the character’s journey back to his creative self from this material world, coupled with a mid-life crisis and a marriage that is going through a transition,” Ruffalo says of the role and his character’s transformation. “There is just a lot of fertile ground.”

Catherine Keener (Capote) plays Dan's estranged wife; the Oscar-nominee Hailee Steinfeld (True Grit) is his rebellious daughter; and the 2012 best-actor Tony winner James Corden (Gavin & Stacey) is Gretta's struggling musician friend Steve.

The Grammy-winning rapper CeeLo Green and Yasiin Bey, formerly known as the hip-hop artist Mos Def, play Dan’s music-biz ­colleagues.

Ruffalo, whose career started in the theatre, would be happy if, like Once, Carney's new film was adapted for the stage.

“I think this could be an easy move,” he says. “So much of the work is already done.”