Begin Again's a first for its stars
Actress Knightley sings in the film and Levine, frontman of the band Maroon 5, acts.
Speaking on the red carpet in New York where the film closed the Tribeca Film Festival, Knightley recalled being "completely terrified" to sing.
And Levine, who said he was "blown away" to be walking a red carpet for a film role, said he enjoyed the experience so much he might have caught the acting bug.
The film opens in the US in July.