Sisters Rooney and Kate Mara learned to speak in unison for Werner Herzog’s drama “Bucking Fastard,” something that came “surprisingly” easily for them.

The film follows two sisters who are so close they “speak in unison, love the same man, and have the same dreams,” according to the synopsis. Herzog, the influential German filmmaker of “Nosferatu the Vampyre” and “Aguirre, Wrath of God,” said at a Venice Film Festival press conference ahead of its premiere on Thursday that he’s been wanting to make the movie for years, but it was only “clear that it could be done” once he met the Mara sisters.

“At first, it was very daunting, the idea that we were going to be playing this kind of same person,” Rooney 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 an unspoken world between us that we didn’t have to work very hard to create at all. I’ve never laughed so much, it was a very once in a lifetime, unique experience.”

Kate added that speaking in unison “felt almost like a song, like we had this unspoken sort of rhythm together.” She continued: “It was very musical, I think. It just sort of happened quite naturally. Surprisingly, it came with a lot more ease than we were expecting.”

Herzog said that the sisters “hardly needed any rehearsals” to get the speaking right. Though they had time for two weeks of preparation, Herzog stopped it after just five days. “I had complete confidence with them,” he said.

“Bucking Fastard” was written and directed by Herzog, who has has a long history at Venice, having premiered 1991’s “Scream of Stone” and 2005’s “The Wild Blue Yonder” 2009’s black comedy “Bad Lieutenant: Port of Call New Orleans” and crime drama “My Son, My Son, What Have Ye Done?” at past editions.

“Bucking Fastard” is loosely inspired by real British identical twins Freda and Greta Chaplin, who became tabloid sensations after becoming besotted with their neighbor, who eventually took out a restraining order on them. The movie’s daring title comes from a spoonerism the twins made (simultaneously, of course) while they were in court. Orlando Bloom and Domhnall Gleeson round out the cast.

“Bucking Fastard” is the first time the Mara sisters are sharing the screen. Rooney Mara is best known for “The Social Network,” “The Girl With the Dragon Tattoo,” “Her” and “Carol.” Kate Mara’s major credits include “House of Cards,” “24” and “American Horror Story: Murder House.”