When it came to choosing actors to portray these historical figures, all of whom were at key stages of their lives when the story took place, specific casting was crucial. As Thomas says, "This is an exploration of the human mind through characters that are young. Jung is thirty, Freud is fifty, Sabina is in her early twenties and Gross in his early thirties. Michael Fassbender, Viggo Mortensen, Keira Knightley and Vincent Cassel were all actors that were desired by David for these roles, and I thought they were magnificent choices."

Sabina Spielrein was one of the first female psychoanalysts, a pioneer in her specialist field of child psychology. Yet, she is barely mentioned in the history of psychoanalysis, despite the fact that in 1912 she presented to the Psychoanalytical Society her conception of the sexual drive as containing both an instinct of destruction and an instinct of transformation. In this presentation there is strong evidence that Sabina influenced the work of both Jung and Freud; from Jung's ideas of archetypes of the feminine in men and the masculine in women (transformation), through to Freud's theory of the sex instinct and the death instinct. Freud later acknowledged in one of his books that Sabina led him to this path of thought, whereas possibly due to the nature of their relationship Jung never publicly acknowledged that her ideas had influenced his thinking.

It was only with the discovery of Sabina's hospital records, her personal journals and correspondence with Jung and Freud, which has now been published, that it became apparent she inspired both men's ideas.

Cronenberg explains what compelled him to bring these complex true-life figures to the screen: "With A Dangerous Method, I sought to make an elegant film that trades on emotional horror, but loses none of its power to seduce. I was stimulated by offbeat and intimate details that illuminate the three leads themselves, and that give a sense of what it must have been like to be at once trapped and liberated by their cerebral and physical bonds. It was a strange menage a trois, not that Sabina had any sexual relations with Freud, but still there was love in each part of the triangle, including between Jung and Freud; there was an incredible affection and friendship between them."

Michael Fassbender – Carl Jung

Michael Fassbender, the versatile and much in demand young actor, was chosen to play Carl Jung, a character he was thrilled to take on as he was intrigued by this relatively unknown episode in Jung and Freud's lives. As he explains, his feeling is that Sabina did indeed influence both these men in their work, "According to the information Christopher Hampton gathered, she did have an influence on Jung in terms of his ideas of introvert and extrovert within personalities, and I really don't think she has been given credit for her influence on both men. There was an interesting dynamic between the three, and Sabina brought to the forefront a sort of rift between the two. But she was also somebody that really wanted them to remain collaborators, because she recognized it could potentially set back psychology 100 years or so if they didn't continue to work alongside each other. What makes A Dangerous Method interesting that it's a slice of these famous people's lives that we weren't aware of, with another little kink or twist to it."

For Michael, the opportunity to work with Hampton's script and Cronenberg greatly appealed to him. As the film unfolds, the dynamic of the relationships between Michael's forward-thinking character and his mentor Freud, his wife Emma, his patients Sabina and Otto Gross who encourage him to cross boundaries, are relationships which many of the audience can relate to. As Fassbender says, "The feeling of the scenes is very accessible because you see they're actually just human beings, doing things to each other that we all do. They have the same lusts and jealousies. There's a lot at play here with these characters. They were brilliant people, but with that comes ego as well. I think that's interesting; people when they're cornered and the reactions they have, and how they deal with people around them, sometimes the closest people to them."

Keira Knightley – Sabina Spielrein

The pivotal role of Sabina Spielrein was offered to Keira Knightley. After reading the script, she immediately undertook the challenge to learn about the world of psychoanalysis with relish, researching and reading books on the subject as well as any information she could find on Sabina, and speaking with psychoanalysts. For Knightley, the opportunity to be a part of A Dangerous Method was one that captured her imagination: "What I found fascinating about the whole story was that it showed the beginnings of psychoanalysis. It's so much a part of our culture, with words like 'ego' or 'complex', that we don't even think about when we use them today. Whereas then it was the very beginning and they were discovering a whole new way of treating people."

The hysterical and troubled Sabina we first encounter is a strong departure from the roles Knightley has played previously. She eagerly rose to the challenge, pushing herself to play this remarkable woman who influenced two of the most influential thinkers of the twentieth century. Knightley immediately saw from Hampton's script how Jung became drawn to Sabina from their first session, in a stark room where he sits behind her and begins practicing Freud's 'talking cure' method on her. As Knightley explains, "Sabina is mentally troubled when she first arrives at the hospital, but I think what takes Jung aback is that she's extremely intelligent and also extremely brave and open. He's been reading a lot about Freud and this form of psychoanalysis, as we call it now, and starts to use her as a sort of litmus test for this 'talking cure'. She's very responsive and I think her honesty, intelligence and her strength, and the fact that she's beautiful, intrigues him and puts him off-balance."

The initial meeting between Jung and Sabina is the beginning of a voyage they embark on together through psychoanalysis. As Fassbender says, "As their relationship develops and he explores this new technique and she tries to figure out her personal and mental issues, they're on a journey side by side, and together they become almost more like colleagues. There's a mutual respect between them in the way Christopher has written it."

Viggo Mortensen – Sigmund Freud

For Viggo Mortensen the opportunity to collaborate with the director for a third time, and play the iconic figure of Sigmund Freud, was one he welcomed. Mortensen completely immersed himself in his study of Freud, approaching the role with the meticulous level of research for which he is renowned, visiting the place of Freud's birth, to his homes in Vienna and London as well as the Burgholzli, reading his books, studying photographs and footage for his look and mannerisms, and even tracking down the cigars he smoked.

Mortensen's study of Freud included his dress and humor. As he elaborates, "Freud continued to dress in the same way for many decades, a nineteenth-century way of dressing. He wrote German the way German was written in the eighteenth and nineteenth century, and he never really changed. There is a formality to his writing and to his presentation of himself. In the way Freud wrote his letters to Jung, there was always a high standard of intelligence in the discourse, but in conversation he was witty and personable. And all of that is fun to play. Jung is a very, very clever man, but very different to Freud."

In his school of thought, Freud believed it was important to tell patients the truth of their situation, and that they had to deal with it. In contrast, Jung felt it important to explain to people the process of how they had become who they were, and to help them on the path to change. Fassbender expands on some of the differences between the two men, "Jung was more open to experiences. For example, he was influenced by the planet and the idea of communicating with spirits, whereas Freud was very much set on one form of psychology: this idea that all neuroses essentially spring from a sexual origin. I think Jung had a problem with that, that it shouldn't be so exclusive."

Mortensen felt Freud's methods varied from Jung’s as he wanted the patient to understand their problem rather than offering a potential solution. He explains, "Freud was basically saying: "We're not really necessarily going to cure people. That can't be the goal. The goal is to listen and understand, in order to help them understand why they do things, why they feel things, why things are as they are, in order to accept it."

It was their differences in approach, opinions and age that Cronenberg found fascinating, "What interested me was to see Jung when he was 29, and Freud when he was 50, in the prime of his career and his intellectual strength. A time when these men were not so familiar to us because they were still forming their theories and bonded through a mentoring relationship. Jung thought of Freud as his father figure, and in fact called him that many times in letters."

Otto Gross – Vincent Cassel

The critical role of Otto Gross was offered to the prolific award-winning actor Vincent Cassel, who had starred alongside Viggo Mortensen in Eastern Promises for Cronenberg, and jumped at the opportunity to work with them both again. For Cassel, this was a chance to play a psychiatrist with a very different approach to Jung and Freud. Gross believed that one needed to fully experience life in order to be able to talk about it within their profession, and lived his life by this conviction.

Gross was modern in his thinking, manner and dress, almost out of kilter for his era. This greatly appealed to Cassel. As he explains, "Otto Gross is really sick. He's into drugs and has a lot of different relationships with many women and kids all over the place. There's one line that really sums up what my character is all about: 'Never repress anything.' So he goes for whatever he feels like. I guess that makes him some kind of a nihilistic character."

Gross meets Jung when Jung is questioning Freud's methods, and also the emotional and sexual feelings Sabina arouses in him. In a sense, Gross is the catalyst for Jung and Sabina's sexual relationship, as it is through their conversations that Jung begins to seriously consider crossing his personal boundaries and violating his doctor/patient relationship with Sabina.

Otto suggests to Jung that he desires but also fears a sexual relationship with Sabina. As Cassel says, "Otto Gross was extremely seductive and also very bright, very brave and bold in the way that he carried out his life and his theories. I think when Jung talks to him in the scenes that we show, he's basically looking for approval and Otto Gross is telling him what he wants to hear. If you take it away from the psychologists' environment it's just two guys together, and I'm giving him advice on how he could behave with this girl. Except that he's a doctor, except that she's sick, except that I'm a doctor and I'm sick too. Which makes the whole thing a dangerous combination."

The attention Jung pays Sabina, and the sexual relationship they embark upon, ignites strong feelings in her. Knightley interprets it thus, "I think at that time, being a woman, being Russian, and also having been through this horrendous illness, I think everything isolated her to a certain extent. I believe she almost certainly saw Jung as her salvation, as the person who had released her and given her freedom. With her sexual complex, which often shifted from one object to another, I suspect it was quite a destructive relationship because that is the pattern of behavior that she fell into and I should imagine if she did transfer this particular complex onto him, that it would have been quite a masochistic relationship."

Emma Jung – Sarah Gadon

Canadian actress Sarah Gadon was cast as Jung's loyal wife, Emma Jung. Gadon found herself intrigued by the passion that ran through the script and the characters and their overlapping relationships with each other, both professional and personal. As Gadon explains, the position Emma held in society and her sense of duty for her marriage and family meant she supported her husband unfailingly, "We meet Carl and Emma near the beginning of their marriage before she gives birth to their first child, and it is really interesting because you get to see the arc of their marital problems when Carl starts having the relationship with Sabina. I think that Emma has a lot of expectations for her marriage in a positive way, but as the story goes on you see how she struggles with the sense of duty. I think her role is really important to Jung because not only did she keep their family life together but she was a support system for him. Without her support he would not have been able to do the work that he was doing."

Women of the era, like Sabina and Emma, are lesser known primarily because they were women rather than men, and generally not encouraged to have an existence outside the domestic sphere. Therefore Sabina's influence in the world of psychoanalysis, and thus on modern day thinking, is all the more remarkable. As Jeremy Thomas says, "There hasn't yet been a landmark film about the extraordinary influence of Freud and Jung on twentieth-century psychoanalysis and A Dangerous Method is just that, focusing as it does on their professional relationship by telling the story of Sabina Spielrein, the woman who came between them."