Damon Herriman’s creepy Charles Manson is haunting viewers concurrently in TV’s “Mindhunter” and on the big screen in “Once Upon a Time . . . in Hollywood” — an experience the Australian actor calls “bizarre.”

“‘Mindhunter’ came about maybe six months before the [Quentin] Tarantino movie, but by pure chance they shot within a couple of weeks of each other,” says Herriman, 49. “Honestly it was just a coincidence. By the time I got the audition for ‘Once Upon a Time . . . in Hollywood,’ I had already spent months and months researching Manson, so it was kind of an unusual situation. But in this case I’d already been doing Manson [on ‘Mindhunter’] so it was very helpful for me that I already knew the character so well by that stage.”

In “Once Upon a Time,” viewers see a wild-eyed Manson knocking on Sharon Tate’s (Margot Robbie) door on Cielo Drive in 1969. He’s looking for record producer Terry Melcher, the house’s previous occupant. Tarantino’s movie puts a fictional twist on what really happened shortly thereafter: the savage Tate-LaBianca murders orchestrated by Manson and executed by members of his “family.”

Herriman appears on “Mindhunter” in Episode 5. The year is 1980 — 11 years after the murders. FBI agents Bill Tench (Holt McCallany) and Holden Ford (Jonathan Groff) interview a bearded, incarcerated Manson, who has a swastika carved into his forehead, to plumb the depths of his twisted psyche. It’s an intense, 10-minute scene — with a nearly unrecognizable Herriman-as-Manson at turns pontificating, philosophizing and bullying — and leaving an ominous inscription in Ford’s copy of Vincent Bugliosi’s book “Helter Skelter”: “Each night as you sleep I destroy the world.”

“They really went all out for the makeup in ‘Mindhunter,’ ” says Herriman. “I think [executive producer] David Fincher has a real thing for getting the physicality as accurate as possible. They used Japanese makeup artist Kazu [Hiro], who’s kind of the best in the world at what he does. He won an Oscar [in 2018] for the Gary Oldman makeup in ‘The Darkest Hour’ and that was the same year we were working together on ‘Mindhunter.’

“Essentially most of my face you see is prosthetic in some way,” he says. “People are kind of like, ‘Wow, how did they find someone who looks that much like Charles Manson?’ Well, I don’t look that much like Charles Manson.”

Herriman says he researched Manson by using archival sources. He avoided watching the many onscreen portrayals of him (at least 12 so far). “I’d watched the original [CBS] ‘Helter Skelter’ telemovie but that was probably 15 years ago,” he says. “I remember watching Jeremy Davies’ extraordinary performance as Manson in the [2004] remake of ‘Helter Skelter.’ He and I worked together on ‘Justified.’ I didn’t go back and look at [the other portrayals] because I just thought, ‘Well, it might get a bit muddy on whether you’re doing Manson or doing someone else’s Manson.’

“I just thought it was simpler to watch the real guy as much as possible,” he says. “And I compiled a little video, eight minutes long, that had as many bits of him that were [as] different from each other as possible because he had so many different versions of himself that he presented. It was a question of how many of those we could show in the [‘Mindhunter’] scene. There was a lot to pack in.”

Herriman chalks up the continuing fascination with Manson, who died in 2017 at age 83, to a few factors. “It’s partly the way he looks, like this frightening, impish demonic character, and then you hear him speak and it’s both creepy and mesmerizing,” he says. “He’s got that cult-leader thing that draws you in. This little 5-foot-2 man had a group of 20-something people following him around and doing whatever he said — and then what they actually did over those two nights is so incredibly horrific.

“It just becomes something that’s hard to shake once you know about it.”

“Mindhunter” Available on Netflix