The 44-year-old writes, directs and stars in Boy Erased, about so-called gay conversion therapy - which involves treating people to try to turn them straight. The practice has been widely condemned, but remains legal in many places. "My greatest hope is that this film is redundant as soon possible," Edgerton tells BBC News. "Whatever we can do to end conversion therapy." Boy Erased is based on a memoir of the same name by Garrard Conley, an American author who wrote about what it was like to go through one of the programmes. Having read the book, Edgerton was keen to adapt it for the big screen, and hopes the resulting film will form part of a wider movement to end the practice. "But in the meantime," he says, "it's a great movie for parents who are dealing with anything akin to what Garrard's parents are dealing with." Edgerton explains it shows "parents getting something wrong in terms of how to accept and love their child", and audiences see "whether they're willing to acknowledge that mistake". In the US, conversion therapy is favoured by often religious parents who enlist their children in the hope of turning them straight. The film is insightful for anyone who has ever wondered what kind of methods might be used. Male participants are seen being given instructions on posture - for example, being encouraged to stand a certain way or not sit with their legs crossed.