For his film Cartel Land, Matthew Heineman embedded himself with a citizen-led paramilitary group in the Mexican state Michoacán.
He admits it was one of the most dangerous assignments imaginable, going undercover with a vigilante group fighting one of Mexico's most violent drug cartels, the Knights Templar.
The cartel supplies most of the methamphetamine to the US, and have been linked with kidnappings,drive by shootings, extortion and the massacre of entire families.
Former California physician Dr Jose Mireles decided to take matters into his own hand, forming a paramilitary group called Autodefensas, to fight back against the cartel. He and his team travel through towns where they supply weapons and training to locals to protect themselves from the Knights Templar.
Matthew Heineman travelled with Dr Mireles and was there for many tense stand offs, both with the cartel, and the army, which tried to stop the vigilante groups.
Matthew Heineman also joined another armed group on the other side of the Rio Grande, the Arizona Border Recon, which patrol Arizona's Altar Valley, known as Cocaine Alley.
He spoke to Kathryn Ryan about the film and how he learned the lines between good and evil are blurred when it comes to the groups fighting the cartels.