Director John Carney’s film, due in 2013, follows a singer-songwriter (Knightly) who forms a bond with a down-on-his-luck record producer (Ruffalo) after she is dumped by her rising star boyfriend (Levine).
Green has been cast as hip-hop star Troublegum, who lends a hand to help the young singer fulfill her musical ambitions.
Hailee Steinfeld and Catherine Keener have also landed roles in the movie.
Carney previously scored an international hit with 2006′s “Once,” which earned an Oscar for its song “Falling Slowly.”
Samuel L. Jackson, right, welcomes Robert Downey Jr. onto the stage during a panel for the movie "The First Avenger" at Comic-Con International Saturday, July 24, 2010 in San Diego.
*The Marvel Studios presentation at Comic-Con Saturday presented surprise guest Samuel L. Jackson to introduce the all-star cast of its forthcoming film “Avengers.”
Just as the company’s panel was winding down, the actor took the stage before some 6,000 fans to bring out stars Clark Gregg, Scarlett Johannson, Chris Hemsworth of “Thor” and Chris Evans of “Captain America.”
He then introduced another surprise guest, Robert Downey Jr., who brought out the rest of the cast, including Jeremy Renner as Hawkeye and Mark Ruffalo as Bruce Banner.
“The Avengers” is based on the comic book series of the same name, which brings various Marvel superheroes together. The film is due in 2012.