"I walked into this dark theatrette where there are just these silhouetted heads, Herriman recalls. "No one said anything. I thought, they're going to be like, hey, Damon, thanks for coming in. [But] no one said anything. I just saw all these heads. I didn't know who they were."

In the acting world, there are two theories about moments like that: that they're trying to help an actor's audition by creating as few distractions as possible, or that they're wanting to see an actor at his most fearful and uncomfortable to see if an actor has the wherewithal for the gig. "It was just silence, and I just said, so, should I start?" he says. "And this one head said, yeah."

Even at that point it had been a long road. The 46-year-old Adelaide-born actor initially auditioned for the Quarry pilot in 2013 in an earlier phase of its development; that version was directed by Australian film director John Hillcoat and he did not get the role. Another pilot directed by Banshee's Greg Yaitanes​ was then commissioned and Herriman auditioned again, and just a few hours after leaving that darkened theatrette, the role of Buddy was his.

Based on a series of books by author Max Allan Collins, Quarry is the story of Mac Conway (Logan Marshall-Green), a Marine who returns home to Memphis from the Vietnam War in 1972 and must grapple with the then-barely understood concept of post-traumatic stress disorder (PTSD) from his experiences in the battlefield, and ostracism from his community because of the unpopularity of America's involvement in the war.

Conway is recruited as a contract killer, where he meets Herriman's character, Buddy, another contract killer, though the two men, Herriman notes, are very different.

"Buddy is perhaps the most layered character on the page that I've ever had the privilege of playing," he says. "He's a gay man in Memphis in 1972, but he is not in any way ashamed of that. He's completely comfortable with that. [But] he also has incredible temper, he flies off the handle to an extreme."

As "a little guy" by his own admission, Herriman says there is a genuine delight in playing "this character who is written to be intimidating, and he intimidates other characters who are actually twice his size. That's so much fun for me."

Over time, Buddy must wrestle with his conscience about the career he has found himself in. "He's been working in this organisation for about five years and despite the fact that he's very good at his job, and that he's perfectly willing and able to carry out his job, he's also struggling with the fact that this is his life."

The setting of the show in a sense makes it a "modern period" piece, much in the same way as a show like Mad Men. The sets, Herriman recalled, opened an unusually interesting dimension to the work and reminded him at times of houses from his childhood.

"I do feel like, to sound a little cliched, that the time and the place feel like another character in the show," he says. "I think the production design is so well done. I was a kid in the 70s, and walking onto these sets is bizarre because it felt like that aunty's house that you used to visit."

The series also resonates in a contemporary sense with its exploration of post-traumatic stress disorder (PTSD), and the issues confronted by a soldier returning to suburban America from an unpopular war. "The show could be set today," he says. "It could be [the story of] Mac suffering from PTSD and coming back from Iraq or Afghanistan."

What is interesting about the post-Vietnam era is that, he notes, the idea of PTSD was largely unheard of then, dressed up by the more generic and less properly understood idea of "shell shock". "There was very little assistance for anyone going through what his character goes through," he says. "Having it set in that time period is important for that reason."

Though Mac and Buddy are very different men, they do share a common characteristic in the sense that both are men wrestling with demons inside themselves and they both, for different reasons, have an inclination to emotional disturbance.

"Mac is a lot more brooding, he's got a darker vibe about him," Herriman observes. "Buddy brings with him a lighter energy, but yeah, he does have this darkness, although I don't think it is because of his sexuality; despite the time, he's struggling more with what he does for a job than his sexuality."

That representation of Buddy – as a man at ease in his own skin – was important to the show's writers Graham Gordy and Michael Fuller, Herriman says. "They felt very strongly that Buddy is really comfortable with who he is," he says.

"It's a different and perhaps unexpected representation of being gay in 1972 in the South, to show a character who is certainly not ashamed of that in any way and doesn't particularly fight it, either. He just doesn't like people making an issue of it or belittling him for it. If they do, there are quite severe consequences."

The role caps off an extraordinary run for Herriman in the US, with roles in Justified – where he played Dewey Crowe for five seasons – as well as Breaking Bad, Battle Creek and Flesh and Bone. In that time, despite amassing an envious body of work, Herriman has retained an element of anonymity, largely because of his almost Gary Oldman-esque ability to transform into characters, revealing little of the real man who lies beneath.

"It's a joy as an actor to do that; it also does give you the freedom to walk down the street," he says. "As long as those opportunities to keep getting work arise, and to keep getting good work, that's really all that matters to me. I think to be able to keep working on good stuff and also have a level of anonymity that means you can also walk down the street and live a regular life, you've kind of got the best of both worlds."

WHAT Quarry

WHEN Showcase, Monday, October 3, 9.30pm