The film, which will be directed by Atom Egoyan, focuses on the trials of Jessie Misskelley, Jason Baldwin and Damien Echols, who were convicted of murdering three eight-year-old Boy Scouts in Arkansas in 1993.
The 35-year-old will star as Pam Hobbes, the mother of one of the victims who at first thinks the men are guilty but then comes to suspect their innocence.
The film will reportedly begin filming next summer.
The West Memphis Three were widely thought to have been wrongly accused of the crimes, and were released from jail in August after 18 years of incarceration.