The actress signed up last year (11) to portray one of the slain boy's mothers in the adaptation of journalist Mara Leveritt's 2002 book Devil's Knot: The True Story of the West Memphis Three, and now executives at Worldview Entertainment, the production company behind the film, have revealed Firth will be part of the cast too.
The announcement was made at the Cannes Film Festival in France this week (ends18May12), although it is not yet known what part Firth will play.
Devil's Knot will be set in 1993, when the three kids were stabbed to death.
Then-teenagers Damon Echols, Jessie Misskelley, Jr. and Jason Baldwin were convicted for the bloody crimes in 1994 and imprisoned. The West Memphis natives, who maintained their innocence, were freed in 2011 after officials agreed there was significant doubt about their guilt.
Miskelley, Jr. and Baldwin will serve as co-executive producers on the project, which will be directed by Atom Egoyan.
Principal photography will begin in Georgia in June (12), according to The Hollywood Reporter.
Witherspoon has yet to confirm or deny reports she is pregnant with her third child, but if the claims are true, Devil's Knot will be her first film role after giving birth later this year (12).