In casting the two main characters, Rebecca Hall said she had to find two actors that could play either role, because both of them are so seduced by and interested in each other's lives. Ruth Negga agreed, suggesting that if they were doing it as a stage play, the actors could trade roles every other night. Tessa Thompson, however, demurred: "I would never want to play Clare. I love Ruth in this part so much, I wouldn't have done it."
The term "passing" refers to the practice of members of minority or oppressed races, religions, ethnic groups, etc., pretending to be white (or otherwise members of the majority culture) to escape prejudice. The writer/director of this movie, Rebecca Hall, is better known as an actress; although she has almost only ever been cast as white characters, she is part African American. In a 2018 interview with Deadline, Hall said, "I came across the novel at a time when I was trying to reckon creatively with some of my personal family history, and the mystery surrounding my biracial grandfather on my American mother's side. In part, making this film is an exploration of that history, to which I've never really had access."
Ruth Negga was instrumental in helping Rebecca Hall get the film made. She had read the book years before and had been blown away by it but was shocked to learn that no one had tried to adapt it up till now. When she heard of Hall's involvement, she pushed hard for it to be made.
Rebecca Hall spent 10 years writing the screenplay. Her first draft took 10 days, the intervening years were spent retuning and refining it.
A month before production began, Rebecca Hall was still $500,000 shy of the $10 million budget that she required. She was obligated to apply for two grants to meet the final figure.