It’s a huge responsibility to bring a version of a character to life who was a real person. Throw in the success of a franchise and an infamous serial killer and Charlie Hunnam knew he needed to get his portrayal of Ed Gein right.

“Monster: The Ed Gein Story,” the eight-part Netflix series, is led by Hunnam as the serial killer who murdered at least two women and exhumed several bodies for his heinous purposes in the 1950s. Raised by an evangelical mother whom he obsessed over, Gein desecrated human cadavers to make a skin suit. His story is rife with disturbing, unprecedented psychological drama and inspired “Psycho,” “The Texas Chainsaw Massacre” and “The Silence of the Lambs.”

Hunnam knew very little about Gein, so when he signed on, he began in-depth research.

“It was initially pretty off-putting, because most of the books that have been published about him are these grotesque catalogs, sensationalizing the darker side of who he was and what he did,” says Hunnam, who was fundamentally interested in “understanding what went wrong in the human being that evolved into the monster.”

That is the essence behind the “Monster” franchise, created by Ryan Murphy. With “The Jeffrey Dahmer Story,” “The Menendez Brothers” and now, with Ed Gein, the series explores true crime through a lens that begs the question: What made the monster, and who is complicit in that evolution?

To understand Gein, Hunnam gained access to his medical records, which he found most helpful to his portrayal. “He was incarcerated in a mental health facility for almost 30 years and did biannual reviews that were transcribed,” he says. “There were thousands and thousands of pages of his medical history available. So I worked my way through them, and that provided a much clearer, more rounded picture of who he appeared to be, at least to me.”

Corey Castellano, the show’s prosthetic makeup artist, worked with prosthetic artist Mark Nieman to map out what that would look like. “We started by taking a life cast of Charlie’s face. On the life cast, we sculpted multiple versions of the hooded lid, which we then tested on Charlie,” Castellano says. “Once we found a version that everyone approved of, we began producing them for daily use in a color that closely matched the lightest tone on Charlie’s skin.”

It ended up being a small appliance made from thin silicone.

“For the actual application, we used a silicone template to ensure that the hooded lid appliance was placed in the same spot each day. The piece was glued on with a medical adhesive, and the edges were blended off with acetone. Once the piece was applied, it was painted using a variety of alcohol-based makeup in thin washes of color to carefully add warmth and match Charlie’s skin tone,” Castellano explains. “For the varying stages of aging, we started by applying the eye and then a forehead, eyebags, crow’s feet, frown lines and nasolabial folds along the side of the nose. The painting process was similar but was finished by adding a bit of silver and gray to his brows to match the wig.”

As small as it was, Hunnam says that the psychology of the hooded eye was important in tapping into Gein. “That was something that Ed was very self-conscious about his whole life,” the actor says. “He was bullied about it at school. People would call him flabby face and flabby eye, and Cyclops, and he would cry at school. There were many elements that led to Ed’s isolation and not becoming socially integrated in the way that ‘normal people’ do.”

Hunnam adds, “By the time Corey and I met, I was so committed to the idea that this was a huge facet that wasn’t just physical — that it informed his psychology so much in the way he interacted with the world.”

“We tried it on, and immediately, I felt like it was transformative. It changed the whole shape, and it was a really important tool,” says Hunnam. “It made Ed feel vulnerable, and with the right attitude, it made him look incredibly sinister.”

Although a recording of the night of the killer’s arrest ended up being inadmissible in court, since Gein hadn’t been read his Miranda rights, Hunnam still used it as a tool in order to learn the cadence of his voice.

“The central thesis for me was a boy who only had one relationship, with his mother, was living in complete isolation outside of that relationship, and if she had told him every day of his life that she hated him because he wasn’t the daughter that she wanted, what would be the manifestation of that?” says Hunnam. “And it seemed plausible to me that he would try to make himself as attractive to her as possible.”

He also read about abuse and the manifestation of abuse in vocal patterns and took important advice from showrunner Max Winkler. “He said, ‘I keep hearing this mother, mother, sound,’ and he did this high-pitched calling of a mother’s voice, and it was a goose bumps moment,” recalls Hunnam. “I said, ‘That’s so crazy, but you’re pointing me in the right direction with that.’ And that’s how it all came together.”

It was important for Hunnam that it didn’t “feel like an affectation that was just designed to make this guy feel creepy or a monster.” Instead, he focused on making sure it was “sincere to the idea of what is the manifestation of this guy’s abuse and how is this going to tragically, deeply affect him — which is the precursor to everything else that goes on to terraform his vocal range.”

Hunnam adds, “He thought if he could become who his mother was, what he perceived his mother always wanted him to be, maybe she would come back from the dead, and he wouldn’t be alone anymore. Which, if you take out the horror of everything that he did, the tragedy of that is too hard to put into words.”