Nia DaCosta and Jack O'Connell on that epic 'Bone Temple' musical number

Critics are raving about Nia DaCosta's 28 Years Later: The Bone Temple, and a big part of why is the absolutely sensational climax, in which Ralph Fiennes channels his inner metal god to perform Iron Maiden's "The Number of the Beast" before an awed Jimmy Crystal (Jack O'Connell) and his remaining Fingers. 

Last year, O'Connell himself stunned audiences with a dizzying collision of horror and dance with Sinners, in which he played the singing, Irish-jigging vampire Remmick. With The Bone Temple, O'Connell counted himself lucky to witness three-time Academy Award nominee Ralph Fiennes transforming from the mild-mannered Dr. Ian Kelson into a fire-twirling Satan, rocking out to an epic song.

In Mashable's Say More episode with DaCosta and O'Connell, the English actor described his reaction watching Fiennes perform this on set as "mindblown," saying, "Here's this legend of the game, fully going for it." 

For more from Mashable's Say More interview, check out the full episode on YouTube.

How did the Iron Maiden dance number in The Bone Temple come together? 

Credit: Sony Pictures

DaCosta reveals this devilish lip sync was in the Alex Garland screenplay, which as a whole made her eager to come on as director. However, reading this particular scene on the page, she thought, "Well, this could end my career. It's like, how do you make this work?"

Across 28 Years Later and The Bone Temple, Dr. Ian Kelson is a bit of an odd duck in the post-apocalypse. He lives in a bunker under a towering ossuary he's made of the bones of the dead. Between that and staining his skin an orangey-red with iodine, he's an intimidating figure at a distance — mistaken for Satan himself by Jimmy Ink (Erin Kellyman) and her sir, Lord Jimmy Crystal. But up close, he's a soft-spoken, educated man, eager to offer comfort and compassion however his skills and dwindling supply of drugs will allow. 

However, under threat of torture and death, the good doctor must masquerade as Satan himself to con the Fingers, as Jimmy demands. Cue Iron Maiden. 

Using the hand-crank record player he's had squirreled away in his bunker and a flurry of remarkable DIY theatrics, Kelson not only awes Jimmy and the Fingers — who admittedly have never seen a rock show of any sort — but also excites the theater audience. 

Both times I've seen Bone Temple (so far), once Kelson's performance concluded, the audience broke into applause and cheers of elation. But staring at the script, DaCosta was aware of how big a swing such a scene is. 

"I think at this point in my career, I've learned to just, like, trust the process and the collaborators," she said. "And that was, like, absolutely that. So Shelley Maxwell, my choreographer; Gareth [Pugh] and Carson [McColl], our costume and production designers; the special effects team; stunts; hair; makeup — like, everyone just came together. And once we [could] see what we want[ed] to do… It was just amazing. And then Ralph having to do that [performance] for hours and be basically naked."

She then deferred to O'Connell, asking, "There was one day where you guys just came at night to see the full thing, right?"

"Yeah," O'Connell concurred, adding, "It was a bit of a crescendo moment. All departments, set design, costumes, a big, big ring of fire and things. They are amazing moments anyway, them crescendo times. But for me, it was very hallucinogenic." 

DaCosta shared his affection for this time on set, declaring, "It was just so insane. It was so fun." She cheered her production team for their detail-oriented work, saying that in the end, they had so much they could "only do the very best ideas." The director added, "I had the best collaborators. Then Ralph killed it. And then the way that Jack and the other Jimmies responded to it was just so — because [their] response is really what the scenes about — like, what he's doing is amazing, but it's really about how these Jimmies are seeing this person." 

For DaCosta, whatever doubts she had looking at the script for the first time were allayed once she saw the first rough cut of Fiennes' dance footage. "On the [shoot] day, I was like, 'That's amazing," she recalled, adding, "When [editor Jake Roberts] sent me that [scene], while we were still shooting, I was like, 'All right, thank God. We're good."

28 Years Later: The Bone Temple is now in theaters.