Emma Watson is a radiant energy in the dark Colonia

Film director Florian Gallenberger with Emma Watson on the set of Colonia. Ricardo Vaz Palma / Majestic

Those who still associate Emma Watson only with the role of Harry Potter pal Hermione Granger are in for a shock.

Her latest film is Colonia, which explores the awful true history of the Colonia Dignidad in Chile. It housed a notorious cult, led by West German Paul Schäfer, and during the military dictatorship of Augusto Pinochet, it was a torture centre.

Watson play Lena, who joins the cult in 1973 in an effort to rescue her husband Daniel (German actor Daniel Brühl) who has been abducted by the secret police locked up in the Colonia Dignidad.

The film was directed by German filmmaker Florian Gallenberger, who won the Oscar for Best Live Action Short in 2001.

“Colonia is a very dark place – if you go there, it takes your energy away,” he says. “I cast Emma because I wanted someone who has a very bright, radiant energy. When you look at her, it is just sunshine.

“I wanted to see the difference between the lightness of her and the darkness of the place.”

The director heard about the cult when he was a child and was angry that the authorities had turned a blind eye towardsit. His fury never relented, and so making a film about Paul Schäfer seemed natural.

Gallenberger visited Chile and met people who had lived in the camp. From the stories they told him, along with researching books and news reports on the cult, he created a film that blended the known fact with a fictitious human story.

“I think, everything that happens in the film, somehow happened to somebody at some time,” he says. “But it didn’t happen to those two invented characters. They were experiences that other people had.”

Brühl was also introduced to the story at a young age.

“When I was a child, we had people from Chile living with us, because my parents helped them,” he says. “They were Chileans, who had fled from Pinochet. So I was interested in the subject matter of the Colonia, anyway, before the film.”

Brühl spoke to some of the former inmates as part of his research for the role.

“There is a strong documentary about what happened, but this one guy, as a living source he was a great help for us,” he says. “He gave us a lot of information, which sometimes was difficult to listen to. Paul Schäfer was a sick psychopath.”

Schäfer was so evil, in fact, that the director had to refrain from showing some of his worst excesses.

“The moments you see are very authentic,” says Gallenberger. “But, I think, life in Colonia was even worse.”

• Colonia is in cinemas now