The stars play college sweethearts who relocate to New York to pursue music careers in the movie, which closed the Tribeca Film Festival on Saturday, and Knightley reveals she was blown away by Levine's acting talents.
Calling the "Moves Like Jagger" rocker a "natural entertainer," she tells the Associated Press, "(Levine was) sensational in this film."
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But Knightley admits stepping out of her comfort zone to try her hand at singing in the movie filled her with dread: "I was completely terrified. I didn't anticipate being as frightened as I was. I said yes to doing it and I was (like), 'Oh, it's gonna be fine, it's gonna be fine,' and then suddenly I found myself in a studio with real people who did it for a living and I was like, 'I don't know what I'm doing. I'm pretending.'"