The film's depiction of C.R. Milne's relationship with his parents is relatively accurate. The actual C.R. never forgave his father for exploiting him (perhaps inadvertently) to sell books. Instead, he learned to accept what had happened and lived with it until he died in 1996. His relationship with his mother was worse; she disapproved of C.R. marrying his cousin. After A.A. Milne died in 1956, C.R. had almost no contact with his mother until she died fifteen years later. C.R.'s only child, his daughter Claire, was born with cerebral palsy. She died in 2012 at age 56.
Although the film doesn't touch on it, in reality, Daphne Milne was mentally troubled and quite badly psychologically troubled by a traumatic childbirth experience.
Both Domhnall Gleeson (who's Irish) and Margot Robbie (who's Australian) took extensive lessons in the post-war upper-class English accent from a dialogue coach. Gleeson would stay in character from morning to night.
Director Simon Curtis was attracted to the screenplay because it effectively depicted the life of one of the very first child celebrities.
To prepare for his role, Domhnall Gleeson did a lot of reading about post-traumatic stress disorder, as well as several biographies of A.A. Milne, including the one written by the real C.R. Milne.