For Knightley, an A-list actress, it was the first time she had to sing so extensively in a film. For Levine, the Maroon 5 frontman, it was his first time acting.
Fortunately, the pair could sympathize with each other when it came to stepping outside of their comfort zones.
"I kept telling her she was great and she wouldn't believe me, and she told me that I wasn't bad at acting either but I didn't believe her, so it was perfect," Levine told The Associated Press at the film's premiere Saturday on the last day of the Tribeca Film Festival.
Though Knightley sang briefly in the 2008 film "Edge of Reason," she had to do far more singing as her character Gretta in "Begin Again." But because Gretta is more of a songwriter than a performer, Knightley said she could get away with not having the perfect voice.
She told Entertainment Weekly that director John Carney "wasn't looking for a voice that was going to carry a musical, and I know I don't have that, but I think you can kind of get away with a lot more in folk music."
She added, "We were kind of going for a talky-songy type vibe that we thought might be quite cool for her, and getting away from anything that was trying to pretend to be perfect."
Casting Gretta's boyfriend and musical collaborator, Dave, who leaves her heartbroken when he becomes famous overnight, required not only a great voice but star-like charisma. Enter Levine, who is also a coach on "The Voice."
"Somebody just pitched in Adam with a bunch of names," Carney, who also directed the 2006 musical sleeper hit, "Once," told EW. "He did a tape and sent it to me, and it was just brilliant."
Of his movie debut, Carney said, "he was so good and is very hard-working, and he doesn't take himself too seriously, which is refreshing."
Added Knightley, "Adam is the classic frontman. He is a ball of energy. You just kind of switch him on and he just goes, and he just bounces off the wall for hours. It's amazing being in a room with that much enthusiasm and that much energy."