Kara Young remembers the fervor around Is God Is’s off-Broadway run in 2018. Playwright Aleshea Harris’s revenge tale ran at New York’s Soho Rep theater from February through May of that year. Young was performing in a different show at the time, but she knew she needed to see Harris’s play by any means necessary.

“I was lucky to get a ticket,” says the two-time Tony award-winning actor, recalling the buzz about the show that rippled through the theater community and saw it transfer to London in 2021. As soon as she saw it, Young easily understood why: “It blew my mind. Those characters have stayed in my spirit since 2018.”

The story is just as moving and unsettling on-screen as it was on-stage. Harris adapted her Obie award-winning show into the new feature film Is God Is and makes her directorial debut with the film, too. The epic tells the story of twin sisters Racine (played by Young) and Anaia (played by Mallori Johnson). As kids, they were disfigured with burn scars after their father set their mother on fire in front of them. The girls moved through the foster system, protecting one another. Racine is the Rough One, as her character’s full name goes. She passes more easily in the world than Anaia, the Quiet One, who wears their physical trauma on her face.

They believed their mother was dead until they receive a letter from her. She’s on her deathbed now, still rendered immobile from the attempted murder. She has one request for her long-lost daughters: “Make your daddy dead.”

“There’s a mythic quality to twins,” Harris explains. Racine and Anaia move between silent and verbose conversations with one another as they wonder whether or not they possess the same capability for violence as their father. The twins felt like a natural pair of protagonists to align with the simple prompt Harris had for this play: if she were to create a Greek tragedy but with people who look and talk like her, what would that be?

“That prompt just opened a thousand doors in my mind,” she says.

Is God Is wasn’t made with the intention of being adapted, but Harris’s original play did draw inspiration from films of female rage, revenge and violence such as Kill Bill and Set It Off. After she was approached by Tessa Thompson’s Viva Maude production company and Janicza Bravo, a fellow producer, (Zola), it was at Bravo’s suggestion that Harris stepped into the director’s chair for the feature.

“There’s so much nuance inside of it and so much humor,” Harris explains. “I can’t even imagine the assignment for someone else, of trying to take this wild, wacky story and keep a hold of its tone, keep it unapologetic and keep its grimy, off-Broadway, punk roots.”

Harris accomplishes that well. The production has a rawness to it as the sisters embark on a road trip through the US south to track down the whereabouts of The Monster, as their father is called. In doing so, they piece together the years their family was apart and gain a deeper understanding of how horrifyingly scary, manipulative and unremorseful this man may be. They also begin to unpack their own capabilities and the divergent dreams the sisters have after decades of just trying to survive.

“There’s a justice around the rage to complete the mission,” Young says of Racine, who has spent her life protecting her more heavily scarred sister from the world’s cruelty. “When we crack her open, it’s really about cracking open the points of no return. It opens a portal into her deepest why.”

Anaia’s sense of rage reveals itself as a deep sense of sadness and loss. Over the course of the girls’ road trip, she’s not as certain that they should be fulfilling their mother’s request. But she still sticks by her sister’s side.

“At the end of the day, [Anaia] just wanted to be a normal girl,” says Johnson. “She just wanted a dad, she wanted a mom, she wanted a good relationship with her sister. and she wanted to feel like she belonged. That was very clear to me from the moment that I read her, but all the complexities of it became much more nuanced as we were working on it.”

Anaia and Racine’s troubled parents have brief, but staggering scenes. Playing their mother is Vivica A Fox, who starred in two of Harris’s biggest film references for Is God Is. The twins refer to their mother as God, believing she is that because she made them.

“I knew that we could call her God and people would buy it,” Harris says of getting her dream star for this role in place. “She has the larger-than-life-ness but also the grounded-ness. She trusted me and didn’t treat me like a baby film-maker.”

“I’m an independent film-maker and first-time director myself,” Fox says, referring to her 2023 directorial debut First Lady of BMF: The Tonesa Welch Story, “so I know what it’s like to be in her shoes.”

Fox recalls Harris telling her she was the first and only choice to play God. In order to transform, she had a 2.30am call time to have the prosthetics fitted all over her body.

“It was a four-hour process of getting into the prosthetics, the wigs, the nails, but it really helped me get into character,” Fox explains. Harris had made clear that God is in her final days, physically tired but excited to see the daughters she has spent years avoiding due to how deformed her entire body had become. “She’s the catalyst for sending them on this revenge mission.”

In contrast, Sterling K Brown’s sociopathic Monster is a chilling inversion of the actor’s heroic leading man image. When his face is finally shown, he masterfully channels the mysteriousness of this man’s motivation and lack of empathy. Harris’s on-screen take on the Monster was something that diverged from the play, wanting to toy with the dichotomy of Brown’s more typically gentle demeanor.

“In the script I wrote ‘he’s like Barack Obama,’” she explains. “It’s giving suburban dad. Sweet, charming, soft. To me, that’s a much more interesting choice. It’s a more layered and delicious choice. I was absolutely thinking about what people think they know about Sterling. I think Sterling had a lot of fun being subversive there, showing another side of him and getting something that was surprising.”

More importantly, the Monster in Is God Is embodies the very real type of abusers that can maintain their social status and good will in society: the ones who can play the good guy even when they’re not.

“The way people who are abusive can get away with it is their charm,” Harris adds.

But the real core of the story is the bond between the sisters. Young and Johnson expertly navigate the complexities of their co-dependence and emerging differences. The two were brought into rehearsals two weeks before filming to do exercises like looking in the mirror to see if they read each other’s minds. They lived in the same apartment and spent 24 hours a day together while filming in Louisiana.

“The core of our connection came from me and Kara being bonded in real life,” Johnson says. It added a softness to the rage, as well as a purpose for seeing through all facets of how it manifests within their relationship to one another and themselves.

“With these characters, you see all of the nuance of why we are angry in the first place, what happens when we go after it, what it does to us, what it does to the people around us, our questioning of ourselves in our rage and the quiet side of the rage,” Johnson adds. “These characters are literal embodiments of all of the nuance of what it means to be a Black woman in America and how we have to navigate ourselves in our journey for our own justice. It’s our own justification for why we deserve more and why we deserve better.”

• Is God Is is out in US cinemas now and in the UK and Australia on a date to be announced