Rooney and Kate Mara speak in unison for Werner Herzog film

Sisters Rooney and Kate Mara play sisters who speak in unison in Werner Herzog’s new film “Bucking Fastard,” which is having its world premiere Thursday at the Venice Film Festival.

The 83-year-old German filmmaker had actually met twins who spoke in unison some 45 years ago. Greta and Freda Chaplin dressed the same and became a bit of a tabloid fascination in the 1980s in the U.K. because of a stalking case. And they did spark an idea, but it was one that lay dormant for decades, constrained by the limitations of reality.

Then someone suggested that perhaps it didn’t have to be twins: What if it were the Mara sisters instead? All of a sudden, it came to life: He wrote the script in five days.

But his story is complete invention, he said. The Maras play the fictional Jean and Joan Holbrooke, who go on a literal and spiritual journey together as they try to find their place in the world.

Herzog was joined on the Lido by the Mara sisters, and Orlando Bloom, who plays the object of both of their affections. The film does not yet have a release date in the U.S.

Rooney and Kate Mara's first film together

“At first it was very daunting, the idea that we were going to be playing the same person,” Rooney Mara said. “But I think we found pretty quickly that having a lifetime together and all this history and a childhood of playing together, there was just this unspoken world between us.”

She added: “I’ve never laughed so much, we had so much fun.”

Kate Mara said speaking in unison almost felt like a song.

“It was very musical,” she said. “It happened quite naturally.”

They were so quick to pick it up, in fact, that Herzog cut the planned two-week rehearsal short after five days. He told the sisters, who have never before been in a movie together, that he’d meet them in Ireland “in front of my camera.”

Orlando Bloom is the bucking fastard

Herzog said he was also excited to work with Bloom and to see him step out of what he’s known for: “The heartthrob for the teenies,” Herzog said. “And now you are the bucking fastard.”

Bloom said that Herzog “creates a sacred space for magic to happen.”

He added: “It’s the pinnacle of any actor’s career to be at the Venice Film Festival talking about a movie with Werner Herzog.”

Herzog dedicated the film to the late filmmaker David Lynch. While they were editing, he said he wished Lynch could be the first audience to see “Bucking Fastard.”

“We had a deep, deep understanding of each other’s work, although we were quite different,” Herzog said. “I wish we could turn the wheel of time back and invite him tonight. But it doesn’t happen like this.”