At the Cannes film festival, Joel Edgerton and Ruth Negga, the stars of Jeff Nichols’s Loving, a biopic of Richard and Mildred Loving, a white man and a black woman who were arrested in Virginia in 1958 for marrying, talk to Nigel M Smith. They tell how their case echoes through to the legilisation of gay marriage in modern America. Nichols explains why he wasn’t keen on making the couple’s story a traditional court room drama