Milly Alcock Once “Sneaked” Her House of the Dragon Costar onto the Supergirl Set — and What Happened Next Had the Entire Crew Watching.

Milly Alcock is entering a new chapter in her career, but it is not one built around the polished certainty often associated with superhero cinema. As Kara Zor-El in DC Studios’ Supergirl, the Australian actress takes on a heroine whose journey is shaped as much by grief, recklessness and emotional conflict as it is by extraordinary power. The role marks a significant transition for Alcock, who first became internationally known as the young Rhaenyra Targaryen in House of the Dragon, and who now finds herself at the center of one of the most closely watched new projects in the evolving DC universe.

The actress filmed large portions of Supergirl at Leavesden Studios in Watford, England, the same vast production hub where House of the Dragon was working on its third season. For Alcock, the overlap created an unusual sense of continuity between the series that changed her career and the film that is expected to introduce her to an even wider global audience. While the fantasy drama continued filming nearby, Alcock was suspended on wires, performing stunts and bringing Kara Zor-El’s complicated emotional world to life.

Ryan Condal, the showrunner of House of the Dragon, was able to witness part of that transformation from a distance. “I saw her flying on the backlot. They were doing this big thing, and I could kind of see it from my office,” he said. “They sent her a picture… Basically, I was just standing in the throne room, like, ‘You’re in the Supergirl costume!’” His reaction captured the surreal nature of Alcock’s rise. Only a few years earlier, she was introducing audiences to the younger version of a future Targaryen queen. Now, she was preparing to become one of DC’s most iconic heroines.

Director Craig Gillespie recalled the scene Condal had seen, describing it as a major confrontation between Supergirl and the Brigands, an all-male group of bandits. Alcock was deeply involved in the physical side of the production and said she was able to do “a s--- ton” of the stunts herself. Gillespie explained that the production took over the largest stage at Leavesden Studios and expanded beyond it. “It was the largest stage in Leavesden,” he said. “And then we built a whole alleyway out the doors.” Alcock added that Gillespie had somehow arranged “eight miles of cable for a massive fight sequence on that stage.”

The demanding work left little room for Alcock to revisit her former House of the Dragon set, even though the productions were taking place so close to each other. As the lead performer in a major DC Studios film, her schedule was tightly packed. Still, she managed to bring a small piece of her former world into the new one. “I snuck Fabien on [the Supergirl] set one day, and he almost got in trouble,” Alcock said, referring to Fabien Frankel, who played Ser Criston Cole opposite her Rhaenyra in the first season of House of the Dragon.

Frankel had apparently attempted to blend in with the crew by pretending to be a familiar makeup artist. “Fabien was pretending to be Natalie, the makeup artist, because Natalie was our makeup artist on House of the Dragon,” Alcock recalled. “But Craig loves to do this amazing thing where he doesn’t cut. So we rolled for like 20 minutes, and Fabien was just there, stuck in that lock-in.” The story offered a glimpse into Alcock’s relationship with the cast and crew who surrounded her during the project that first placed her on the international map.

For Condal, seeing Alcock’s continued success has been a source of personal pride. “I’m very, very proud of her,” he said. “It’s amazing what’s happened to her in her career. I take great pride in it.” The sentiment reflects the affection many viewers developed for Alcock after her performance as young Rhaenyra Targaryen. Although her time on the series was limited to its early episodes, her portrayal helped establish the emotional foundation of the character’s later story and became one of the most discussed elements of the show’s first season.

Alcock’s Kara Zor-El is not designed to resemble the clean-cut, endlessly composed superhero audiences may expect. Based on Tom King and Bilquis Evely’s Woman of Tomorrow comic run, the film presents Supergirl as a character who is bruised by her past and often uncomfortable with the expectations placed upon her. The word “messy” has frequently been used to describe this version of the heroine, and that complexity is central to the project’s tone.

When audiences first meet Kara, she is partying across the galaxy on planets orbiting red suns, whose solar energy weakens Kryptonians. It is the only environment where she can experience the effects of alcohol and drugs. The choice immediately separates this Supergirl from more conventional depictions of the character. Instead of beginning with an image of perfection, the film introduces a woman trying to escape the pain she carries with her.

Gillespie was drawn to the sharp tonal contrasts in Ana Nogueira’s screenplay. “I read the first two scenes that Ana had written and the extremes are there,” he said. “The opening scene is very intense.” He was referring to the moment when Krem of the Yellow Hills, played by Matthias Schoenaerts, murders the family of Ruthye Marye Knoll, portrayed by Eve Ridley. The tragedy sends Ruthye on a mission of revenge and eventually brings her into Kara’s orbit.

“And then we go into this very irreverent drunken scene with Milly’s character,” Gillespie continued, “but there’s a lot of pathos in it, there’s a lot of pain, and then there’s humor, and then there’s a recklessness. All of that’s in this opening scene, and I was immediately in.” The director’s description suggests a film that aims to use the superhero format as a setting for a more intimate emotional story, rather than relying only on spectacle.

The comic book run that inspired the film has often been compared to True Grit, with Ruthye asking Supergirl to help her track down and kill Krem. There are also echoes of revenge thrillers in the story. During their first confrontation, Krem poisons Krypto, Kara’s super-dog, leaving Supergirl with only three days to find the antidote. The journey that follows becomes both a physical pursuit and an emotional reckoning.

The film’s darker undercurrent is rooted in Kara’s own history. Unlike Superman, who arrived on Earth as a baby and was raised by the Kents in Kansas, Kara remembers Krypton and carries the trauma of watching her entire world disappear. Her grief is not simply a piece of background information. It shapes the way she responds to danger, intimacy, responsibility and the idea of becoming a symbol for others.

“The biggest battle that she’s had to face is with herself,” Alcock said. “There’s something incredibly accessible within that; there’s something incredibly relatable within that. I think that audience members will feel that this is a superhero that reflects who they are as opposed to who they’re told to be. I think that that’s really refreshing.” Her comments reveal why the role may be particularly suited to her strengths as a performer. Alcock has built much of her early reputation on characters whose emotions are not easily contained by the worlds around them.

The casting process for Supergirl reportedly came down to Alcock and Meg Donnelly, who had previously voiced Kara in several DC animated projects. Other actresses said to have been considered included Cailee Spaeny and Emilia Jones. Alcock met with Peter Safran, co-head of DC Studios alongside James Gunn, toward the end of 2023. At that point, she was told that the studio was considering a Supergirl film, before returning to Australia and later receiving the call asking her to submit a self-taped audition.

For her audition, Alcock recorded two very different scenes. One was the bar sequence in which a drunken Kara defends Ruthye from a violent man. The other was a flashback set on Krypton, where Kara speaks Kryptonian with David Krumholtz, who plays her father, Zor-El. The latter scene included four pages of dialogue in an invented language, a challenge that immediately impressed Gillespie. “I was shocked when she came in that day,” the director said.

Gillespie explained that the combination of Nogueira’s screenplay and Alcock’s audition made the project feel fully realized. “It was a tone that I knew that I was very comfortable with, and particularly knowing that it was going to be Milly in that role,” he said. “In her work, I’ve seen [those] complicated, emotional, accessible performances that can be very funny and tragically comic at times. She does such a beautiful, nuanced job with that.” His assessment reflects the qualities that made Alcock stand out in House of the Dragon, where Rhaenyra was both privileged and vulnerable, impulsive and politically aware.

Alcock’s experience on House of the Dragon also proved useful in unexpected ways. David J. Peterson, the linguist who helped develop High Valyrian for the Game of Thrones universe, also worked on Kryptonian for Supergirl. “I feel like Kryptonian was harder,” Alcock admitted. “I feel like High Valyrian was more Portuguese or Spanish. There were a lot of rolled Rs. There was a natural cadence that came out. High Valyrian felt like a different set of muscles, but the process of learning was the same.”

The physical demands of the role were another adjustment. Alcock had already spent time performing dragon-riding scenes on House of the Dragon, but she found solo flight in Supergirl more difficult. “Wire work’s just brutal,” she said. “There’s something really constricting about being in a harness and hanging. You can’t stand or sit very comfortably. If you have to go to the bathroom, you have to take off the costume and the harness and then the under costume. So it’s just very fussy.”

Before the global attention that came with House of the Dragon, Alcock had been building her career steadily in Australia. Born in Sydney, she began appearing in local television productions as a teenager and gained attention for roles in series including Upright, where she starred alongside Tim Minchin. The show gave her an opportunity to demonstrate the understated, emotionally direct style that would later become one of her defining qualities. She also appeared in Australian dramas such as The Gloaming, Reckoning and Fighting Season, gradually becoming a familiar face to viewers in her home country.

Her casting as young Rhaenyra Targaryen changed the scale of her public profile almost immediately. When House of the Dragon premiered in 2022, Alcock’s performance drew praise for giving the character a sense of volatility, intelligence and loneliness. Her Rhaenyra was not introduced as a polished future ruler but as a teenager navigating family expectations, political pressure and a growing awareness that her life would never fully belong to her. The role placed Alcock in the center of a major franchise at a relatively early stage in her career, but she managed to make the character feel personal rather than merely symbolic.

Her departure from the series after the time jump did not end audience interest in her work. Instead, it created a demand to see what she would do next. Alcock went on to appear in the Netflix series Sirens, further showing her ability to move between prestige drama, dark humor and emotionally unpredictable characters. The transition to Supergirl now places her within another major franchise, but the project’s tone appears to offer her more than a standard superhero introduction.

Away from the screen, Alcock has generally kept her private life out of the public conversation. Unlike many young stars who become closely associated with highly visible relationships, parties or social-media controversies, she has maintained a relatively restrained public image. Much of the attention around her has remained focused on her work, her acting choices and the speed of her professional ascent. That privacy has allowed her performances to remain at the center of her public identity, even as her fame has grown.

The absence of major public scandals has also contributed to the perception of Alcock as an actress navigating rapid success with caution. She has spoken in the past about the sudden changes that followed House of the Dragon, including the experience of moving from Australian productions to international press attention. Yet her approach has remained notably grounded. She has not attempted to reinvent herself through a highly curated celebrity persona, and her interviews often return to the practical realities of acting: learning dialogue, working with directors, handling difficult scenes and adjusting to the physical demands of a set.

That grounded quality may become especially important as Supergirl reaches theaters. DC Studios is in the middle of reshaping its cinematic universe under James Gunn and Peter Safran, and the film will carry expectations beyond Alcock’s individual performance. Kara Zor-El has appeared in films, television series, animation and comics for decades, but this new version is being positioned as a more emotionally raw and unpredictable figure. The challenge will be to honor the character’s legacy while allowing audiences to meet her as someone distinct from the Supergirl they believe they already know.

For Alcock, the role arrives at a moment when she is no longer simply the breakout performer from House of the Dragon. She is becoming a leading actress whose choices are beginning to define her own path. Her Supergirl is not presented as a flawless savior, nor as a character who exists only to complement Superman’s story. She is a survivor, a reluctant guide, a grieving daughter and a woman whose strength is inseparable from her imperfections.

As Supergirl opens in theaters, the film offers Alcock the opportunity to bring a new kind of superheroine to the screen. The audience may recognize the cape, the Kryptonian mythology and the familiar name, but the character at the center of this story is built around something more unsettled. Through Kara Zor-El, Milly Alcock is stepping into a role defined not by certainty, but by the difficult process of learning how to live with everything that has been lost.