Florian Zeller wanted Sir Anthony Hopkins specifically for the part. He sent Hopkins the script in 2017 and waited for a reply. In the meantime, he did not pursue production with any other actors in the lead role. He said if Hopkins had not agreed to the film, then it likely would have been made in French instead.
Anthony says his date of birth, December 31, 1937; this is Sir Anthony Hopkins' actual date of birth.
Sir Anthony Hopkins won the Best Actor Academy Award at age 83 for his role in this film, which made him the eldest winner of an acting Oscar, beating the previous record set by Christopher Plummer, who had won the Best Supporting Actor Oscar at the age of 82 for Beginners (2010). The record for oldest actor to win an Oscar for a lead role was previously held by Jessica Tandy, who was 80 when she won for Driving Miss Daisy (1989).
Sir Anthony Hopkins was so sure that he was going to lose the 2020 Best Actor Oscar to the late Chadwick Boseman that he didn't attend the awards, opting to stay in his native Wales. He reportedly asked the producers to make an appearance via Zoom, but the producers declined the offer. Against tradition, the Best Actor award winner was announced last, after the awarding of the Best Picture winner. The show's producers later admitted that this was done because they also were expecting Boseman to win, and wanted the ceremony to end with his family tearfully accepting the award. Hopkins even had gone to bed early, missing the announcement that he had won (Announcer Joaquin Phoenix accepted the award on his behalf). After waking up and hearing the news, Hopkins quickly released an acceptance speech on Instagram, in which he admitted that he "really did not expect this," and paid tribute to Boseman.
The play itself is quite dialogue-heavy, so Florian Zeller and Sir Christopher Hampton's biggest task when adapting the screenplay was to pare down the amount of talking in the film. This is reflected in the multiple scenes of Sir Anthony Hopkins looking around him in bewilderment. Instead of having the character speak, they instead relied on the actor's facial ability to tell the story.