Cillian Murphy says 'Oppenheimer' came out the same day as 'Barbie' partly because Christopher Nolan is superstitious

• Greta Gerwig's "Barbie" and Christopher Nolan's "Oppenheimer" were released on July 21.
• Cillian Murphy told Margot Robbie that Nolan was "determined" to have a summer release.
• "He has this superstition around that date, the 21st," Murphy said.

"Barbie" star Margot Robbie and "Oppenheimer" actor Cillian Murphy discussed the pop-culture phenomenon known as "Barbenheimer," and explained how filmmaker Christopher Nolan's superstition influenced the release date of his movie.

Robbie, the titular star of Greta Gerwig's "Barbie," and Murphy, who played J. Robert Oppenheimer in Nolan's "Oppenheimer," sat down together as part of the latest installment of Variety's "Actors on Actors," released on Tuesday.

During their conversation, the actors spoke about "Barbenheimer" — the moniker used to describe the rivalry between "Barbie" and "Oppenheimer," both of which were released in theaters on July 21.

"Christopher Nolan was always determined that it would be released in the summer as a big tentpole movie," Murphy said when asked if he anticipated the response to the biopic about the father of the atomic bomb.

"That was always his plan," he continued, "and he has this superstition around that date, the 21st."

Murphy said that Nolan's movies are usually released "in and around the 21st of July."

"It's a good date," Robbie replied.

Nolan made his feature-film debut with the movie "Following," which was first released in April 1998 as part of the San Francisco International Film Festival. His subsequent early films, such as "Memento," had varying release dates.

"The Dark Knight," the second film in Nolan's "Batman" trilogy starring Christian Bale, was his first movie to be released in his preferred slot of mid-July. Long before "Barbenheimer" overtook the internet, Universal Pictures's "Mamma Mia!" competed against "The Dark Knight" on July 18, 2008.

Since then, a majority of Nolan's movies have been released in that month: "Inception," "The Dark Knight Rises," "Dunkirk," and "Oppenheimer."

Back in July, before the release of "Barbie" and "Oppenheimer," sources told Business Insider's Jason Guerrasio that Nolan wasn't pleased that Warner Bros., the studio that he previously worked with for years, opted to release "Barbie" during the same weekend as Universal Pictures's "Oppenheimer."

According to sources, efforts were made to get Warner Bros. to change the release of "Barbie," to no avail.

Robbie, who produced "Barbie," told Murphy that a member of the "Oppenheimer" team called her and suggested moving the release date of her film to avoid the two movies competing, but she stood her ground.

"One of your producers, Chuck Roven, called me, because we worked together on some other projects," Robbie recalled. "And he was like, 'I think you guys should move your date.' And I was like, 'We're not moving our date. If you're scared to be up against us, then you move your date.'"

Robbie said that Roven was insistent on "Oppenheimer" keeping its intended release date.

"And I was like, 'We're not moving!'" she said. "I think this is a really great pairing, actually. It's a perfect double billing, 'Oppenheimer' and 'Barbie.'"

"That was a good instinct," Murphy told Robbie.