The 27-year-old is tipped to play Seth's screen girlfriend, who leaves the mild-mannered farmer after he backs out of a gunfight. He then meets the wife of a notorious outlaw, played by Charlize, who teaches him how to fire a gun.
Seth will once again sit in the director's chair for his second film, which has been written with his Ted co-writers Alec Sulkin and Wellesley Wild.
Production on the film is scheduled to begin in May.
Amanda, who played Cosette in Tom Hooper's Oscar-nominated musical remake, has recently been promoting the Linda Lovelace biopic Lovelace and Mark Webber's drama The End Of Love, which also stars Michael Cera.