This season, everything changes when his children grow up and suddenly the question of human sexuality becomes a very real and present issue.

For Michael Sheen, the 46-year-old Welsh actor tasked with bringing him to life, the dilemma is particularly intriguing. "I think the idea of sexuality, which requires a loss of control, which requires you to give yourself up to something, is very frightening to him," Sheen says.

"The idea of keeping it academic and scientific and ordered and structured is very helpful to him, but it's not his true nature," Sheen says. "I think that's a big part of what fascinated me about Bill from the beginning, was seeing a man who has created a kind of illusion of order."

Masters of Sex was based on the book Masters of Sex: The Life and Times of William Masters and Virginia Johnson, the Couple Who Taught America How to Love, by Thomas Maier. It was adapted for television by Michelle Ashford and is now entering its third season.

With growing children, Sheen notes, "the challenges of parenting are slightly changing. Now I have to deal with my own son who's older and he's now around the age that [my character] was being abused by my father. That presses a lot of buttons, I think."

One of the key reasons Masters was drawn to his scientific work, Sheen says, is because he feels he is someone unworthy of love. "If he can get recognition through his work then that sort of gives him a validation that he doesn't feel is open to him in life."

That issue becomes even more complicated as Masters and Johnson's work is revealed to the public, a turning point in the story reached as the second season drew to its conclusion.

"Once you become a public figure, obviously, it's not just about work; it's not just through the medical establishment, it's the world," Sheen says. "It's about how people respond to you and I think that absolutely terrifies him because I think he sort of sees himself as a monster. [He thinks] if people see who he really is they will see him for the monster that he really is."

The third season opens with a jump in time that places the series' narrative in the middle of the 1960s. But Sheen is cautious about trying to interpret events through the prism of the sexual revolution which, to some extent, defined that decade.

"You have to remember we're in St Louis," he says. "It's true, the sexual revolution happened in California and New York. It was different [in St Louis]. In terms of when you talk about the sexual revolution there's lots of different versions of that going on. I think it was a slower change for the majority."

Though the series began its narrative in the 1950s, there are powerful statements contained within it which reflect on modern society and modern sexuality. Though Sheen is cautious about making those observations from the perspective of someone working on the show.

"That's not for me to say, really, that's for the people who are watching it to say.  I think from working on it, it certainly feels like what we're working on is not some sort of period piece. It's not just resigned to the past. It feels very resonant to our lives today, certainly to my life today."

As an actor, he says, he has no interest in playing  in a museum piece. "What was exciting about the project in the first place was to be able to do something through the lens of history that actually is about now, is about what we're going through now," he says.

Sheen has never seen his career  as "work", with diverse portrayals, from a trilogy of projects as former British prime minister Tony Blair (2003's The Deal, 2006's The Queen and 2010's The Special Relationship), as the comedian Kenneth Williams in 2006's Fantabulosa! and as David Frost on stage in 2006's Frost/Nixon, and in 2008, in the film adaptation of the play.

"It's what I love doing, it's how I navigate my way through the world, it's how I understand things about myself and about the environment I'm in," he says. "I've never really seen it as work. I've been very fortunate. I think you start to see it as work once you don't have it."

Through the show's lifespan, Sheen has added the role of a co-producer to his bow, saying the collaborative aspect of the series is perhaps the most satisfying for him.

"When you're working on a film you get the script and there's a certain amount that you can work on that and change things and bring things into it," he says. "With this, which is ongoing, I actually get to be able to work out story lines for Bill, to say maybe we can do this, [or] I'd really love to explore this aspect of him now.  That's been the most exciting aspect of doing this."

Masters of Sex, Thursday, SBS, 9.30pm