Coming out of nowhere in 2006, writer-director John Carney’s Once was a demo-tape revelation of a movie, the understated alt-pop antidote to a scourge of prefab overblown musicals. Carney’s newest film seems in many ways like a glossed-up Once remix, with songs less good, actors more famous, and moments of truth mostly in quotation marks. It’s the same basic stuff: a romance consummated chastely through the making of a record, this time with Keira Knightley as a woebegone singer-songwriter and Mark Ruffalo as the washed-up producer she inspires. Together, fleshing out her songs and recording them live in various outdoor New York City locales, they seek correlation between heartache and hooky melody, and ponder the everlasting tension between creative integrity and mainstream success. While stubbled, rumpled Ruffalo gets reduced to cigarillo-waving shtick, Knightley mists her eyes and bites her lip as only she can. “I think that you lost the song in the production,” her character eventually tells the fame-spoiled boyfriend who left her, a sensibly cast but perfunctory Adam Levine, and it almost sounds like a filmmaker’s note to self. Whether lost or not, Begin Again is song-like, a fantasy story with familiar specifics. At least it’s nice to know Carney’s still a big believer in the romance of wearing your headphones and wayfaring through a city at night. How else but by being completely sincere could he not seem to notice that treating the quest for authenticity as a formula tends to be self-canceling?