“I got halfway down a 20-meter pool and stood up and actually thought, ‘I can’t do this. I don’t know what I’ve done. I shouldn’t have agreed to it,’” Ridley said during a recent video chat from Los Angeles. “I was really fully learning a new skill.”

Ridley promptly plunged into an arduous training schedule, swimming three to four times a week for several months under the guidance of Siobhan-Marie O’Connor, a two-time Olympian for Great Britain. After Ridley flew to Bulgaria for the start of filming in April 2022, she embarked on a rigorous two-week regimen to round out her preparation. Once production began, she complemented her days of swimming on set with practice sessions during evenings and weekends. Along the way, O’Connor helped hone Ridley’s technique through fingertip drag drills, no-kick and kick-only laps, and myriad other exercises.

When it came time to shoot the scenes from Ederle’s milestone moment — a 21-mile swim from France to England on Aug. 6, 1926 — Ridley had progressed enough that director Joachim Rønning said he barely called upon the movie’s stunt swimmers over a week-plus of filming in the Black Sea.

“I just ended up wanting to use Daisy as much as possible,” Rønning said. “The way she swam it, nobody could really beat it. So even for wide shots when there’s no one way to see that it’s Daisy, I just ended up using Daisy all the time — and thank God she was up for it.”

Nearly two years after wrapping production, “Young Woman and the Sea” is hitting theaters Friday. Adapted by screenwriter Jeff Nathanson from Glenn Stout’s 2009 book of the same name, the Disney-produced drama shines a light on Ederle, the New York native who won two individual bronze medals and was part of the gold-medal 4×100-meter relay for the United States at the 1924 Olympics in Paris. Two years later, she finished her historic swim in 14 hours, 31 minutes, besting the men’s record by nearly two hours.

Although the accomplishment was heralded at the time, Ederle, who died in 2003 at age 98, is no longer the household name she was when her feat of endurance made global headlines and prompted a ticker-tape parade through Manhattan.

“I just feel so, so lucky to be able to retell the story for a modern world,” Rønning said, “and put the Trudy Ederle’s name back on the map.”

When the movie introduces Ederle, a second-generation German immigrant who was advised not to swim after childhood measles hampered her hearing, she’s a plucky teenager with a thirst for swimming but a lack of formal training. In portraying Ederle’s years-long journey from amateur to elite, Ridley mapped out a swimming evolution that begins with a sluggish doggy paddle and ends with the bent-arm stroke Ederle used to traverse the Channel.

While replicating an Olympian’s fine-tuned technique may seem daunting, Ridley said Ederle’s earlier, rawer swimming scenes threw her for more of a loop.

“The doggy paddle, honestly, was one of the hardest things,” Ridley said. “Whenever Siobhan asked us to do doggy paddle, I said, ‘Please, can we not do doggy paddle? I’ll do it on the day. I just don’t need to practice.’ It was so difficult.”

Suppressing any temptation to re-create the English Channel with a water tank or green screen, Ronning always knew he wanted to film in the open water. For Ridley, shooting in the Black Sea came with concurrent priorities: To swim fast enough to keep up with the boat filming her; to maintain the distance from the camera necessary to stay in focus; and, of course, to actually act and convey Ederle’s blend of joy, fear and determination.

The cold temperatures — Rønning said the water hovered around 60 degrees during filming — heightened the challenge. To boot, Ridley endured a strained chest muscle during filming she said still bothers her, as well as an osteoma and an infection in her left ear.

“It’s always challenging making a movie,” Rønning said. “But then when you put it on the ocean where you literally have so many moving parts, it does make it more complex. It’s hard and it’s stressful and I have an actor in the water whose teeth are [chattering], but I think in the end it’s worth it.”

“Young Woman and the Sea” also focuses on the societal hurdles Ederle overcame at a time when access to pools remained restricted for many women, female swim teams were severely underfunded and some officials actively undermined female athletes. (Jabez Wolffe, Ederle’s coach played by Christopher Eccleston, was alleged to have intentionally sabotaged her first attempt to cross the Channel.)

Fittingly, the film is coming out during a watershed moment for women’s athletics. Caitlin Clark is taking basketball by storm. The likes of Katie Ledecky, Simone Biles and Nelly Korda are bracing for the Olympic spotlight this summer in Paris. The England women’s soccer team (a.k.a. the Lionesses) is developing a fervent following in Ridley’s native country. But as the actor pointed out, many of the obstacles Ederle faced remain relevant in a race for athletic equality that’s far from finished.

“It is wonderful that the Lionesses did what they did and Caitlin Clark did what she did,” Ridley said. “But there are also so many people who still do not have the visibility they should and do not have the pay that they should. So it’s an ongoing conversation … and hopefully [the film] continues to help push the conversation forward.”