The film stars  Elle Fanning as a teen who is preparing to transition from female to male, seeking the support of her mother Naomi Watts and grandmother Susan Sarandon. After premiering at the Toronto Film Festival in 2015, the producers have held onto it for a year and a half, partly due to some controversy over the casting process.

Naomi Watts and Elle Fanning in 3 GenerationsNaomi Watts and Elle Fanning in 3 Generations

"Were I to be making this film today," admits director-cowriter Gaby Dellal, "I absolutely would be casting a trans kid. But finding a kid that hadn't medically transitioned or hormonally transitioned yet and was in exactly the same time frame as the character of Ray is incredibly hard to find. Elle did a terrific job. She worked very, very hard to make the character as authentic as possible."

Indeed, Fanning was determined to honour the truth of the situation. "There is so much heart and soul in it," she says. "And I feel that it's a story that people really need to see and should be told, especially now. There was an LGBTQ club at my school, and I knew a lot of trans kids, so it struck me and it seemed like a very authentic story because it's not just about that, it's also about the whole family dynamic."

To prepare for the role, she spoke to a number of trans boys and watched their stories on YouTube videos. "There are a lot of videos there," she says, "little journals that each day they kind of go on. It's nice that they have this community where they can find other people like them or relate to someone who is going through the same things that they're going through. Some days they'd be feeling great, others were awful. Maybe feeling good that someone called them 'sir' that day - that makes them feel better!"

Fanning was struck by one particular video. "It was about a trans man who had been taking testosterone," she explains. "And the video was a time-lapse, showing them getting an Adam's apple, their voice getting deeper and a beard growing in. And then from the start to finish you could see how happy they gradually were. And at the very end how ecstatic it was just that basically what was living inside them for so long finally got to be shown on the outside, and how amazing that was. It was very emotional."