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.
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.
The apocalyptic dream, described by Jung may refer to the beginning of World War I (1914-1918). During World War I Jung was drafted as an army doctor and soon made commandant of an internment camp for British officers and soldiers. Swiss neutrality obliged the Swiss to intern personnel from either side of the conflict who crossed their frontier to evade capture. Jung worked to improve the conditions for these soldiers stranded in neutral territory; he encouraged them to attend university courses.
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 said faces.
Matthias Schoenaerts auditioned for a role, the audition went well and the casting agents wanted to call him back, however, he had pledged his commitment to Bullhead (2011) and so had to let the opportunity slip.