Some true Hollywood heavyweights have been involved with David Cronenberg's new psychological drama A Dangerous Method. Oscar winners Christian Bale and Christoph Waltz were on board at one point, and when they left to take other projects, the master Canuck director landed Keira Knightley, Michael Fassbender and Viggo Mortensen

But the director cautions that there's more to casting than just selecting a few of the hottest stars du jour.

"Casting is very difficult and I think of it as a black art," Cronenberg said during last September's Toronto International Film Festival.

"It's actually one of those talents that a director must have to be a good director, and yet it's invisible. Most people don't really think of that when they think of directing but you can kill your movie before you start by miscasting it badly."

It would appear Cronenberg made all the right picks for A Dangerous Method -- which explores the genesis of psychoanalysis with Sigmund Freud (Mortensen), his pupil Carl Jung (Fassbender) and the mostly unknown Sabina Spielrein (Knightley) -- given the lauded performances from its stars and the scene-stealing turn by Vincent Cassel (Black Swan).

And yet Cronenberg was actually forced to scramble after Bale and Waltz, originally in line to star as Jung and Freud, backed out to take other roles.

Waltz's departure was particularly sudden and led Cronenberg to reach out to his trusted collaborator Mortensen, who had previously been considered for the role of Freud.

Their work together on A History of Violence and Eastern Promises made it an easy decision, Cronenberg said, even if some might not have considered Mortensen an obvious choice for Freud.

"I wanted to deliver a Freud that was not everybody's idea of the Freud we all know, Freud when he was frail in his 80s," Cronenberg said.

"This is Jung when he was 29 and Freud when he was 50 and at the height of his power. He was very charismatic, very masculine, handsome, seductive, forceful and witty."

He was relieved to find his final trio of Mortensen, Fassbender and Knightley had the right chemistry to evoke the "intellectual menage à trois" the psychiatry pioneers shared, and the powerful curiosity about human nature that Cronenberg shares.

"Centred around all of these characters is an obsession with the truth about the human condition, and it's something which an artist is also looking for," he said.

"I think a psychoanalyst and an artist do somewhat similar things in driving to delve beneath the surface of things to see what's really going on underneath."

Knightley spent months studying Spielrein, who first met Jung as a patient and later had an affair with him. It was Jung's fascination with Spielrein's condition -- hysteria, which leaves her afflicted with tics and uncontrollable outbursts -- that prompted his first correspondence with Freud.

Knightley's manic depiction of Spielrein may strike some viewers as overwrought, which wouldn't necessarily surprise the actress, who says she pushed herself to deliver a Cronenberg-esque performance.

"I don't know with another director that I would've taken it as far as I did because I don't know that I'd have trusted them but ... I had complete trust that if I was going too far he'd pull me back," she said.

She refined her performance -- including the "physically quite difficult" jutting of her jaw to replicate Spielrein's tics -- in front of a mirror for days before auditioning her ideas to Cronenberg via a Skype video chat.

"I talked to some psychoanalysts about her and they sort of describe that kind of behaviour ... as a kind of desperation to release some of her pent up emotion, some of that pent up energy that can't go anywhere," Knightley said.

"I wanted it to be shocking ... which I kind of felt like it was my job to do."

Mortensen passed on playing Freud at the first opportunity due to illness in his family, but he admits he also wasn't convinced he was ideal for the part.

"It's not something I would've thought of that I would be right for, probably most people wouldn't ... but once I started researching it I realized I had preconceptions which many people have of the guy," he said.

Mortensen was intrigued as he learned more about Freud's life and personality, and jumped at the chance to play him after poring over his dialogue in the script.

"I don't usually get to play characters that speak a lot, they're mostly non-verbal or a lot of the information you get about the characters and their thoughts on things, their points of view, their attitudes are non-verbal," Mortensen said.

"And in this case he doesn't stop talking. It's his defence mechanism, it's his offence, his defence, his evasive tactics are all verbal, it's his way of charming, it's everything. Words are the gestures more than the physical."

Aside from a few sex scenes between Fassbender and Knightley the film is far more staid than much of Cronenberg's prior work and Mortensen applauded the director's straightforward approach to telling the story.

"I think most directors -- even really good directors -- taking on the subject would've made all kinds of efforts with their camera work to show off, as a way of showing the importance of the subject matter," he said.

"David just had a lightness and an easy, graceful way of shooting this movie. He had a great script, he could just shoot it in an elegant way and concentrate on the drama."

The film is surprisingly compact -- it runs just over 90 minutes -- given the weight of the source material but Knightley said it's a drama first and a little psychology lesson second.

"The arguments and the ideas that are in it are very, very clear and fascinating but they're spare. (The film) is not going into a huge amount of detail -- it's there, the knowledge of it is there because we all knew what we were talking about -- but it is a drama."