Bridget Jones’s beau Colin Firth is joining Reese Witherspoon for a movie about the West Memphis 3, “Devil’s Knot”.

Atom Egoyan (“The Sweet Hereafter”) directs the pic, based on a script by Scott Derrikson and Paul Boardman. Boardman’s now working with Egoyan on a rewrite.

Deadline says of Firth’s role :

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.

“Devil’s Knot” has been in the works for about five years; Derrikson and Boardman originally set it up with Dimension. The film’s producers are now scouting for a new home.

Firth and Witherspoon previously worked together on “The Importance of Being Earnest”.