SPOILERS AHEAD for Joker.
One of the biggest revelations in Joker was that Joaquin Phoenix’s Arthur Fleck’s relationship with Zazie Beetz’s Sophie was not real and was all inside his mind. This, according to Beetz, was not set from the start and Sophie’s role and story actually evolved and changed during filming.
During an interview with THR, Beetz was first asked to discuss this big reveal and if, upon a second viewing, there was anything we should look for that “distinguishes Arthur’s Sophie form the real Sophie?”
Source: IGN.com Joker: How Zazie Beetz's Sophie Character Changed During Filming