When “War Pony” debuts in Cannes on May 21, it will represent the culmination of a project almost a decade in the making. Directed by Riley Keough and Gina Gammell, and written by Keough, Gammell, Bill Reddy and Franklin Sioux Bob, the film about two boys growing up on the Pine Ridge Reservation in South Dakota took seven years from conception to post-production.

Protagonist Picture is handling international sales on the project while CAA Media Finance is repping North America.

“It was a very slow burn,” Keough says of the film. “Because we didn’t really have a clear intention for many years.”

Keough met Reddy and Sioux Bob, who both grew up on the Pine Ridge Reservation, when they were extras on Andrea Arnold’s 2016 feature “American Honey,” in which Keough  starred. When a scene featuring the trio got delayed by six hours, they had nothing else to do but hang out, becoming fast friends in the process.

Keough then introduced Reddy and Sioux Bob to Gammell, another longtime friend (one so close Keough describes their friendship as “kind of like twins; we finish each other sentences”) and soon the foursome, all in their early twenties and “interested in making art,” began experimenting with music, films, VR shorts, music videos and writing.

“And one day we realized we had a story going,” Keough recalls. “A full narrative that could be finessed into something that made sense. So then at that point, I think we started taking it more seriously.” Informed by Reddy and Sioux Bob’s own experiences growing up on the reservation, the script continued to evolve and by 2019 Keough and Gammell found themselves directing a movie.

“War Pony,” which Keough describes as “a really big learning experience for everybody involved,” took three years to wrap, interrupted by the COVID-19 pandemic as well as “crazy snow and weather.” The film was shot almost entirely on location, and while Keough describes the crew as “amazing,” she felt the responsibility of “bringing a bunch of people from outside into a community.”

“The most challenging part I think is life stuff,” says Gammell. “We’re working within a community and in people’s homes. Life doesn’t stop when you’re shooting a movie. We had birth, death, marriages, incarceration. We kind of went through it all over the seven years.”

As well as the “very stop-start” nature of the production, there were also “a lot of logistical challenges,” says Gammell. “We had to fight really hard the whole way,” she says. “From financing it to […] the logistics.”

“I think that the nature of this film has been like, oh, we have a 120-page script, how the fuck that that happen?” says Gammell. “And then we’re shooting the movie. It’s like, do we really have the money? It’s really happening? There was never a definitive green light, everything was so organic.”

All of which was brought home when the duo found out last month — just days before the line-up was officially announced — that “War Pony” had been selected for Cannes’ Un Certain Regard strand. “It was one of the most flooring, surprising, overwhelmingly joyful moments of my entire life,” says Gammell.

Keough also says she was “shocked” by the news. “It was the first time I’ve ever thought I was gonna faint,” she recalls. “I lay down and I had to breathe.”

In a twist of fate, Keough also has a connection to another film premiering at the festival, Baz Luhrmann’s “Elvis,” which stars Austin Butler as Keough’s maternal grandfather, Elvis Presley. “I think it’s totally magical that my film’s there while ‘Elvis’ is there,” she says. “I adore Baz and I have seen the film. I actually watched it a couple of days ago and it was a very intense experience. And I’m very excited for the world to see what Baz has done and what Austin’s done.”

For Keough and Gammell, the focus right now is on Cannes – which they’ll be attending with some of the team from Pine Ridge – followed by more writing, more directing and more producing. The duo have set up a production company, Felix Culpa (Latin for “happy fall”) and are already thinking about their next project (“War Pony” is a Felix Culpa and Caviar production.) “There’s many ideas percolating,” says Gammell.

While “War Pony” is rooted in reality, Keough says the directing and producing partners “love all genres of film” and would consider something totally different, tonally, for their next project, including sci-fi and fantasy. “We’re most interested in human beings and relationships,” Keough says. “And that I think will be a through line.”