Phoebe Waller-Bridge and her longtime friend and creative partner Vicky Jones once made a joke pact that if they ever found themselves in a sticky situation, such as a dead-end relationship or a dull social gathering, one of them would whisper to the other, “Run,” and they would drop everything and make a dash for the hills together.

“It wasn’t something we really did — that would have been embarrassing,” Jones tells Variety, “but it was an idea that we nurtured, that made us feel safe. There was always someone you could run away with, someone you would rather be with than anyone else in the world.”

Fast forward a few years, a handful Emmys and a successful theatre company later, and the duo decided to take the pure escapism of their pact and “Run” with it. Their new HBO series, created and written by Jones with Waller-Bridge in an executive producer and supporting on-screen role, tells the story of Ruby (Merritt Wever) and Billy (Domhnall Gleeson), who made a similar promise when they were in college: If one of them texted, “Run” and the other texted back, they would meet on a platform at Grand Central station and embark on a new adventure together.

“Run” takes place largely on a train hurtling across the country, with plenty of flashbacks and hints at the characters’ regular lives. The show rattles along at 100 mph, and the changes in tone are similarly speedy. What starts out as a “Before Sunrise”-adjacent love story, hurtles towards becoming a slapstick comedy, and eventually an outright thriller.

“We wanted to keep you on your feet at the same time as capturing something more real,” says Jones. “‘Before Sunrise’ did the romantic part so well, but we wanted to go into all that detail and minutiae of a relationship, and then have the world come crashing in as well. They’re running from reality as much as towards each other, and that reality needs to come and bite them on the ass at some point.”

Interestingly enough, both Wever and Gleeson agreed that the tone of “Run” changed radically not only episode-to-episode, but also from the show they first started making.

“When we originally shot the pilot I thought it was going to be a lot more like ‘My Dinner With Andre’ on a train, but with a lot [more] sexual tension,” Wever says. “It shifted a lot, and many of the things that I put roots down or had started to hang my hat on as guide posts for Ruby shifted.”

Gleeson says that the “genre elements,” including the “Strangers on a Train” thriller vibe, were what attracted him to the project — and what he felt set it apart from a straight down the barrel romantic comedy.

The train setting lends a clear sense of confined urgency to the central relationship, and Gleeson recounts that shooting a lot of those scenes made him feel as off-balanced as his character.

“Shooting on the train was great, but very difficult. It was in a studio and there were screens outside that were hooked up to a camera which had a sensor on it that told them which direction it was looking, so the perspective of the image outside the windows would change depending on where the camera was, but that’s different than where your eye is,” he explains. “It messes with your balance, with your sense of not throwing up your lunch. That sucked; that was not good.”

Mixed into the thriller is a story about former lovers who are still drawn to each other, raising the stakes and the tension.

The premiere episode sees Ruby and Billy take turns masturbating in the train toilet (“You have to be pretty turned on to do that,” Jones points out). “Run” shares a common trait with “Fleabag” in its unabashed approach to depicting characters’ sexual urges.

The roaring sexual tension between Ruby and Billy, which many reviews of the show have alluded to, ebbs and flows so agonizingly by design, Jones says.

“It felt true that you would have this raging drive to have sex and that would dissipate and come back. It wouldn’t all go perfectly the first time. I’ve seen too many scenes where the sex looks amazing. It doesn’t matter how much you fancy someone, it doesn’t always happen like that,” she says.

The reasons Ruby and Billy have for running away are complicated and will get explored as episodes unfold. But similarly, their feelings toward each other, so many years removed from each other’s lives, are complicated, as well. They have each settled into adulthood and become slightly different people than who they were when they were in college together. For Ruby, Jones specifically wanted to play with the trapped, unhappy housewife trope.

“We felt passionate about telling a story about a woman who could love her home life and at the same time turn her back on it. People will ask why would a middle class, normal mother with love in her heart and a great life on the surface, walk away from it all? It’s a truth that needs to be told. It’s a taboo that a lot of women think about doing it,” Jone says. “It’s about someone who might not have the worst life in the world, but at the same time is incredibly frustrated, who feels miles away from herself, who has a strong sense of anger and disassociation with her life. The one thing she can do to change things is to walk out.”

“Run” premieres April 12 on HBO.