Richard Gadd is fast establishing himself as the master of uncomfortable viewing. If “Baby Reindeer,” the super-viral 2024 Netflix series he wrote and starred in, raised difficult questions about masculinity, power and sexuality, his follow-up, “Half Man,” does the same — on steroids.

The HBO/BBC co-prod, which ends its six-episode run on May 28, follows the lives of two boys in Scotland in the 1980s, who are thrown together as teenagers when their mothers develop a relationship. Nerdy Niall (played by Mitchell Robertson as a teenager and Jamie Bell as an adult) and volatile Ruben (played by Stuart Campbell and Gadd) are polar opposites. But with the passage of time, their relationship mutates from enmity and abuse to friendship and brotherhood. There is even, perhaps, a hint of incestuous desire. HBO boss Casey Bloys wasn’t kidding when he described it as “intense.”

Like “Baby Reindeer,” the protagonists of “Half Man” are both deeply dislikeable yet strangely sympathetic. In the hands of Campbell and Gadd, Ruben, the personification of toxic masculinity, is endowed with a pathos that becomes compelling, even captivating. (Some viewers may well end up rooting for him over Bell’s increasingly self-pitying Niall). But he’s also, as the show reminds us repeatedly, abjectly dangerous.

“There’s two different types of Ruben,” says Gadd. “There’s the calculated version of Ruben, and then there’s the explosive and in-the-moment version of Ruben as well.” Both are lethal.

Still, it’s not only audiences who are drawn to Ruben. Throughout the series, Niall cannot seem to let his stepbrother go, calling him at the first opportunity when he moves away to college, leading to devastating, life-changing consequences. And despite his bluster, Ruben is equally drawn to Niall. “I think there’s a constant intertwining of protectiveness and possessiveness with Ruben, in terms of his relationship to Niall,” says Campbell.

That much is clear right from the premiere, in a challenging scene where Ruben not only pushes Niall to lose his virginity — procuring a woman to have sex with him and forcing her onto him — but stays to coax him through the deed. “It was a lot scarier on the page than it actually was to shoot,” Campbell says of the episode, which required two intimacy coordinators. “For me, that scene reinforces to Niall a lack of autonomy.”

Given the show’s themes, it’s unsurprising that considerable attention was paid to the cast’s emotional and mental well-being. Julie Cullen, who plays Niall’s college flatmate Joanna, says the show’s team of intimacy coordinators encouraged the actors to “check in with each other,” particularly after an intense scene. “We had a lot of people who were helping us through anything that we needed,” Cullen says. “We did need it.”

Bonds also formed between the cast members, particularly Campbell and Robertson, which helped with some of the more emotional scenes between the young actors. Campbell, who, in the second episode, has to aggressively and repeatedly kick Niall’s friend’s head in (in real life, he used a sandbag), says his relationship with Robertson was integral to accessing the depths of rage and emotion he needed to play Ruben.

“[We] became quite close friends quite quickly,” he says. “I could feel safe with him to be vulnerable and to challenge myself and to go to the places that the relationship and that the script requires.”

The relationship between Ruben and Niall, however, is less straightforward. As well as being friends and stepbrothers, there is also a sexual, even incestuous, undercurrent that is never explicitly addressed. When asked what he made of it, Robertson says it “wasn’t something that I played intentionally.” For him, Niall’s obsession with Ruben stems from awe and fascination, a theme that Bell also carried into his portrayal.

But Gadd, who wrote the series, acknowledges it’s there. “I suppose it is an undercurrent,” he says. “I think their relationship is innately complicated. It’s idolatry mixed with love mixed with hatred mixed with adoration and need. I think they spent their whole life intrinsically bound and yet not fully knowing the depth of their feelings for one another, and that’s what I wanted to explore. I don’t think they even know just how much they love one another.”

Each episode is bookended by the present day, in which Niall is celebrating his wedding to Alby (played by Bilal Hasna as a college student and Charlie de Melo as an adult) when Ruben, freshly out of prison, turns up uninvited. The scenes, which were shot over three days in a barn in a remote part of Scotland, are increasingly menacing, reaching a crescendo in the season finale. Although Gadd is keen to keep spoilers under wraps, the audience already knows, thanks to an Episode 4 reveal, that Ruben is going to die. Whether he takes Niall down with him remains to be seen.

Shooting the denouement between the two men was physically grueling, Gadd recalls, and the schedule was tight, meaning they could only do a couple of takes. “It felt like one of those things where you had to take a deep breath and just absolutely go for it,” says Gadd. The ending is likely to be polarizing, not least because it leaves a number of questions unanswered.

But, for Gadd, that’s what made it compelling, saying, “I don’t think happy endings, or even conclusive endings, are really true to life.”