In 2001, while conducting interviews for their 2003 film "The Globalisation Tapes", Oppenheimer and Cynn began delving into the Indonesian mass killings of 1965–66. After moving up the ranks of those involved with the killings, Oppenheimer's interviews led him to meet Anwar Congo in 2005. The film was shot mostly in Medan, North Sumatra, Indonesia, between 2005 and 2011. After seeing an early preview of "The Act of Killing", filmmakers Werner Herzog and Errol Morris signed on as executive producers.