Colin Firth has joined Reese Witherspoon in Devil’s Knot, the Atom Egoyan-directed drama about the West Memphis 3, reports Deadline. The Sweet Hereafter and Chloe director Egoyan came aboard to direct a script that was originally written by Scott Derrickson and Paul Boardman, the team behind The Exorcism Of Emily Rose.

Firth will play Ron Lax, a private investigator who was the first pro bono supporter of the defendents as they headed to trial in 1993. Lax built one of the most prominent private investigative firms in the Southeast, and offered his services for free to the defendants, who at the time were reviled because of the heinous nature of the crime and the sensationalized reports about devil worship and ritualistic sacrifice. All of that was later proven to be unfounded, after Damien Echols, Jason Baldwin and Jessie Misskelley were convicted despite not a shred of physical evidence. Lax was an unsung hero and helped find the DNA in the knots that bound one of the victims, that cast suspicion on Terry Hobbs, the stepfather of Steven Branch, one of the 8-year old murdered boys.

Witherspoon will play Pam Hobbs, the mother of Branch. She initially believed the trio murdered her son, is eventually persuaded that the three suspects are innocent and wrongly accused. Filming begins next summer in Louisiana, and other big names are expected to join the cast in smaller roles.

Egoyan has been working hard with Boardman on a rewrite. The script is based on investigative reporter Mara Leveritt’s 2003 book “Devil’s Knot: The True Story of the West Memphis Three,” an in-depth chronicle of the sensationalized trials that sent the three to prison for the murder of the three 8-year old boys who were found hog-tied in a drainage ditch.