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The film's cast also includes Green's fellow "The Voice" coach, Adam Levine.
Production stars Monday in New York on the Exclusive Media project.
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Green will play Troublegum, a "very successful hip hop star," Exclusive Media said.
"Can a Song Save Your Life?" stars Knightley and Levine as a couple that moves to New York to pursue their passion for music. After Levine's character dumps Knightley's character, she encounters a down-on-his-luck record producer (Ruffalo) who assembles a ragtag music team to appear on a record with her, including Green's character Troublegum.
Carney wrote and will the direct the film.