Julianne Moore readily admits that she isn’t a big podcast listener—she’s more likely to turn on an audiobook when she’s in the car. But she knows how to spot a good story, which is what happened when she read the script for Case 63, a fiction podcast from Spotify that hit number one on the audio platform shortly after its debut nearly two weeks ago. “It was so compelling,” Moore tells Vanity Fair. “I loved the mystery of it. It’s deeply romantic and futuristic and scary. I just really thought, Yes, I’d love to do this.”

The thriller series also gave her the opportunity to step into the recording booth with longtime friend Oscar Isaac. “He’s insanely gifted as well as being a lovely human being, so it was easy,” Moore effuses. The feeling is mutual. “Julie is a friend, a masterful actor, and someone I’ve been desperate to work with for a very long time,” says Isaac. “The fact that we could be in the room together, playing off each other with such mysterious and sexy material, was just so much fun. Made my desire to do a movie with her all the more burning.”

Case 63—which is based on a Spanish-language podcast that became a popular pandemic listen in Latin America—relies on the chemistry of its two leads. There’s no faking it: The two-hander tells the story of psychiatrist Eliza Knight (Moore), who is assigned to treat a patient known as Case 63 (Isaac), a man who claims to be from the future. Over 10 episodes that play out largely through recordings of their therapy sessions, Dr. Knight’s assumptions and allegiances are challenged as her patient, who calls himself Peter, reveals more about himself. To create the intimacy needed between the two characters, director Mimi O’Donnell asked Moore and Isaac to record the podcast across from each other, much the way their characters would have been conducting their sessions. “We could look at each other and kind of experience it,” says Moore, adding that they recorded the show in a matter of days under O’Donnell’s sure hand. “We actually went much faster than I anticipated.”

Moore and Isaac, who are also executive producers on the series, have to do a lot of heavy lifting on Case 63, and they don’t have a lot of run time to do it—the entire series clocks in at less than two hours, and the audio-only format means that there’s a good amount of exposition to help set the scene for listeners. Additionally, because Case 63 is a science fiction story, they have to help the listener understand that the story’s world, while very similar to our world, has a few key differences, including that whole time-travel thing. In many ways, Moore’s character is the audience proxy, the person who goes from being grounded in reality to questioning her long-held assumptions about her life. “She’s skeptical at first, but she really begins to believe there’s just too many coincidences,” says Moore. “That’s always really compelling to me.” Isaac, meanwhile, embodies a character whose motivations are less clear. “The writing is so strong, and the nature of how Peter is revealed throughout the show—he’s a mystery needing to be solved—allowed me to just trust the script and the dynamic that Julianne and I created together,” he says.

Isaac is no stranger to podcast work. He voiced one of the lead characters on the 2016 thriller Homecoming, which hailed from Gimlet, the same podcast studio that produced Case 63, and was later adapted into an Amazon television series starring Julia Roberts, Stephan James, and Bobby Cannavale. Isaac calls working on Homecoming “a great reminder of the power of dramatic storytelling through audio presentation—like the old radio shows that this country was raised on.” When Case 63 came to him, he says, “it seemed like a great opportunity to come back to the format with a new, interesting story.”

Both Isaac and Moore had early indications that Case 63 could become a hit. The original Chilean series—Caso 63, created and written by Julio Rojas—was the top podcast in Argentina and Mexico for nearly a month after its premiere and trended in several other countries, including the US. Brazilian and Indian adaptations of the series charted in both of those countries as well. All episodes of the English-language version, adapted by Mara Vélez Meléndez, dropped on October 25. Two days later, it became the number one podcast in the US on Spotify. It has also topped the charts in Canada, Ireland, and the UK.

Case 63’s early performance indicates that listeners will likely get more seasons. The original series consisted of 30 episodes spread across three seasons, and in terms of the US version, Moore says she believes “the plan is that it continues.” If that happens, she’s looking forward to getting back into the recording booth. “It was such a fun, creative experience,” she says. “It’s something I want to do again.”