A Dangerous Method stars Viggo Mortensen, Michael Fassbender and Keira Knightley.(Liam Daniel, ©Sony Pictures Classics, courtesy Everett Collection)

Viggo Mortensen in "A Dangerous Method" (Sony Pictures Classics)

After fulfilling title prophecy as a small-town family man with a hidden past in master Canadian director David Cronenberg's adaptation of "A History of Violence" and going further again in collaboration as a Russian mob accomplice in "Eastern Promises", teaming up with Cronenberg again to play father of psychiatry Sigmund Freud must have struck Viggo Mortensen as comparatively dry.

But in speaking about "A Dangerous Method" -- and as he shows in the intellectual and sexual currents of the period drama - taking on Freud is anything but.

"Now, he's starting to make a comeback," notes Mortensen, seated on a windswept terrace near the lakefront during the film's Toronto International Film Festival premiere week, "because there are MRIs and scientific evidence -- now that people can see more of the brain and what's going on with unconscious activity -- that actually he was right. A lot of his ideas that people had been putting down for a long time have been vindicated recently. So, there's a resurgence. But, you know, it ebbs and flows."

Mortensen's Freud is agile of mind, body and a very insidious humour, taking in young protégé Carl Jung ("Shame" and "Hunger" star Michael Fassbender), a surprisingly stuffy idealist who starts to diverge when confronting his own moral place in treating a lovely but virtually incommunicable woman named Sabina Spielrein (Keira Knightley), whose sexual response to acts of punishment throw both Jung and Freud into a seemingly unknowing crisis of what psychiatry is and where it is going.

"Until recently, though, for decades, Jung has been much more in favour. Particularly among artists," Mortensen - an extraordinarily engaging and thorough conversationalist - says. "You know what I mean?... Jung seemed to be much more open-minded in mythology and all that.

"In reality - and I had this idea, too, you know, before I took on the role and started really researching it - what little I'd read about [Freud] and from what I'd read of his work, which wasn't a lot, but some, I had this concept of him visually that he was this frail old white-haired man, and that personality-wise and ideas-wise that he was extremely formal, very rigid, inflexible... and I found the opposite to be true in every respect. Particularly in the period that the movie's about -- his early years, before he got cancer and got really sick and ended up looking the way that we normally think of him.

"He was very robust," Mortensen elaborates. "He was really energetic. He was funny. He had a big appetite for life. He liked to drink wine. He liked to have nice food. He liked to travel; he traveled a lot. He was fascinated, as interested and as knowledgeable about mythology as Jung was. His study was full not just of statues, but books about Ancient Rome and Egypt and Greece. It was very important to him. So, it wasn't like he was against that."

The differences begin to appear, though, once Jung confronts not just the dilemma he sees with psychiatry simply as a science -- an establishment, "A Dangerous Method" shows, that Freud and his peers were doing their utmost to firm up in a skeptical world -- but also in dealing with the mutual sexual attraction between the young doctor and his searching, grasping patient. The growing battle between the two men is less of a gauntlet-dropping showdown customary to most films, and more a test of will and thinly concealed emotions, as the elder Freud employs a certain sly wisdom to counter a very buttoned-down Jung -- who is tightly ordered cold to the point of near cruelty to his wife (Sarah Gadon) and mother of his children at home -- who grows ever more headstrong while still seeking some sort of permission to admit to and act on his impulses.

"There's a lot of dialogue," Mortensen says. "I've never had the pleasure of playing a character that spoke so much. Usually, I play characters, including for David, who express themselves much more through physical gesture than verbally. In this case -- once I crafted and felt comfortable with this and had this knowledge about him, and being someone who spoke with irony and was a great conversationalist -- then I got more relaxed. Then, I thought 'Well, it's gonna be fun.'

"It seemed a great obstacle. I think it's always that way. The thing that seems like the biggest obstacle - the thing that you think 'Aw, shit, how am I gonna do this?" -- once you make friends with it and get comfortable with it, it becomes your favourite thing about the project. And I loved it. Once I got used to it, I loved using words. I loved speaking and trying to find a way -- even though he doesn't really tell any jokes outright, there's an irony. To find a contrast between the more staid, austere, Lutheran type of guy, and then this Viennese, Jewish, urbane guy, It's really fun to find that contrast. And the way to do that was with words."

In fact, Mortensen appears on screen to revel his role in "A Dangerous Method". There are several spars of wonderful wit, including Jung's first appearance at the Freud home on invite to a family dinner, in which the younger doctor is so self-involved that it's as if he doesn't notice several other people awaiting food around the dinner table -- and, furthermore, a feeling that Jung halfway doesn't care that there are.

"Offence, defence; seduction, evasion," Mortensen notes of the dialogue. "The more you get into this particular story, the more they talk. And the more they talk, the less they seem to be saying about what they're really thinking, you know? It's like they use words to avoid talking about what they really need to talk about."

While those accustomed to a more bloody simmer to collaborations between Mortensen and Cronenberg may be wondering what they're doing in the middle of all this, "A Dangerous Method" may, in so many ways, be their most taut and riveting collaboration yet. And after two or three decades of young students toting their paperbacks of Jung's "Man and His Symbols" about as if they were some sort of holy text, Fassbender's Jung and Mortensen's Freud are a welcome and necessary break with the recent past.

"I think he also saw Jung coming," Mortensen says of Freud, "knowing that he was a bastard son and from a family of bastards. 'Ah, I could see what could happen here. This guy has this spiritual bent. He's gonna twist what we're trying to keep a science into this mystical thing. It's like each thing in its place.'

"And he was right, you know, as it turned out. Jung became more of a religious figure - as much a religious figure as a scientist, in a way, you know. Kind of a spiritual guru-type guy for a lot of people. A lot of what people say about both of them either can [be] or are huge exaggerations in terms of both of them. What's nice about this movie is that it's a script that's even-handed - academic, but historically accurate without becoming some kind of documentary. It's fun."