The age difference between Viggo Mortensen and Michael Fassbender is 19 years, just as it was between Sigmund Freud and Carl Jung.
In preparation for his role as Sigmund Freud, Viggo Mortensen read everything he could find on Freud and even visited his actual home (now a museum) in Vienna. In an attempt to capture aspects of Freud's personality, Mortensen smoked his own Danish grandfather's cigars that he kept in his grandfather's personal cigar case over the course of the shoot.
David Cronenberg (an atheist of Jewish heritage) said that he had to teach Viggo Mortensen to "walk like a Jew."
According to Keira Knightley, at first she didn't know how to play her character's hysteria. When she read some of Spielrein's notes she noticed the woman described her condition as being like "a demon or a dog."
Knightley then started to pull faces and contacted David Cronenberg through Skype to show him the results until they both agreed on one.
Keira Knightley joked on The Graham Norton Show that her first meeting with David Cronenberg was a Skype session where she had to demonstrate her character's "weird sex faces" (which she had been practicing in the mirror beforehand). Skype apparently froze while she was pulling one of the faces.