The film, being produced by United Artists and distributed through MGM, is based on the 1968 My Lai massacre that took place during the Vietnam War. Mikko Alanne wrote the script.
Behr will play Lt. Stephen Brooks, the commanding officer in charge of the GIs at My Lai. Gigandet will play Fred Widmar, a soldier who is wracked with guilt and shame after witnessing the slaughter that day.
Behr and Gigandet join Bruce Willis, Channing Tatum, Woody Harrelson, Michael Pena, Xzibit and Toby Jones, previously cast in the film.
Michael Pitt is being eyed for the part of the infamous Lt. Calley, who was found guilty of ordering the massacre.