Sir Laurence Olivier took the role of Dr. Szell in part to leave a great deal of money to his wife and children, as he expected to die from the cancer that afflicted him throughout production. He performed the role while undergoing treatment for his cancer, which included heavy doses of painkillers to allow him to work every day. The pain medication affected his memory, and at times Olivier could not remember more than one or two of his lines at a time. In a testament to his fierce concentration, his performance garnered rave reviews and an Oscar nomination, and despite working under such aggressive medical treatment, he experienced a full recovery, allowing him to enjoy the success of this movie, and a series of leading roles that followed.
Producer Robert Evans was set upon getting Sir Laurence Olivier to play the role of Dr. Szell. However, because Olivier at the time was riddled with cancer, he was uninsurable, so Paramount Pictures refused to use him. Desperate, Evans called his friends Merle Oberon and David Niven to arrange a meeting with the House of Lords (the upper body of the British parliament). There, he urged them to put pressure on Lloyd's of London to insure Britain's greatest living actor. The ploy succeeded, and a frail Olivier started working on this movie. In the end, not only did he net an Oscar nomination for Best Actor in a Supporting Role, but his cancer also went into remission. Olivier lived on for another thirteen years.
Dustin Hoffman lost fifteen pounds for this role. He ran up to four miles a day to get into shape for playing the role. He would never come into a scene and fake the breathing. According to Producer Robert Evans, Hoffman "would run, just for a take, he would run for a half-mile so he came into the scene, he would actually be out of breath."
On the last day of shooting, Sir Laurence Olivier visited Dustin Hoffman at his house, bringing with him "The Complete Works of William Shakespeare" as a gift. He then proceeded to read scenes from several of the plays, much to Hoffman's delight. Hoffman credits the story about his conflict with Olivier to general malice on the part of Writer William Goldman, who did not take kindly to the fact that Hoffman had persuaded Director John Schlesinger to change the ending of Goldman's book.
Although not particularly a fan of William Goldman's original novel, Dustin Hoffman took the role so that he could work with John Schlesinger again (the two had previously collaborated on Macadam cowboy (1969)). He had also heard that Al Pacino was interested in the role and wanted to beat him to it.