– The Promise is a sweeping romantic epic in the tradition of Dr. Zhivago, its lavish budget denoted by its stars, Oscar Isaac and Christian Bale. It includes a scene unlikely to be equalled in importance this year. It is 1915, and Mikael (Isaac) has slipped back through lines of marauding Turkish troops towards his home village, hoping to rescue his family. Instead, he finds the villagers piled like rubbish by a river, the female corpses’ headscarves a futile effort at modesty. The wooded setting could be a Belorussian forest in 1941, in one of the souvenir photos Nazis snapped of the Jewish Holocaust.

But these are Armenians, the Christian minority who lost 1.5 million to systematic extermination by the Ottoman Turkish government in World War One. The term “genocide” was coined by Jewish lawyer Raphael Lemkin in 1944 to describe the Armenians’ destruction, when its pre-echo of ongoing Nazi slaughter was clear. And yet this is the first time a major film has shown audiences what happened. After 102 years, its visceral impact finally pierces the silence.

Isaac, who made his name as the failed folk-singer anti-hero of the Coen Brothers’ Inside Llewyn Davis (2013), and found true fame as dashing, sexually ambiguous X-wing fighter pilot Poe Dameron in Star Wars: The Force Awakens and its upcoming sequel, felt the scene’s impact when he read it.

“I was incredibly moved every time I would go back to it,” the 38-year-old says, speaking with soft fluency in a Manhattan hotel room. “I had questions about certain other aspects of the movie, but every time I would read that scene, it would never not affect me. That was one of the big reasons I wanted to do the movie – to try to understand how a moment like that could happen, and to figure out how I would get myself to have an at least somewhat honest reaction to it.”

Isaac’s preparation for playing an Armenian villager who leaves for cosmopolitan Constantinople to be a medical student in 1914, only to be almost drowned by history’s tide, involved deep research amongst LA’s Armenian community, and in the genocide’s copious archive. “What was particularly useful,” he explains, “was listening to recordings of older gentlemen speaking many, many years after the fact about what they witnessed as children. Seeing their grandmother stabbed to death by the gendarmes. Little babies being laid by a tree and left there. Being marched out to the desert. All these different kinds of images that you read about, so they became very personal.”

Isaac entered an almost meditative state as the crucial, draining scene approached. “I just came on the set and tried to feel quietly concentrated, but not overly focused, and listened to music. So you’re in a state of relaxation, and ready to respond. Doing that scene felt like it did when I read it.”

Michael’s doctor is an unusual, quietly decent hero, reminding Isaac of people almost as close to home. “There’s a gentleness to him,” he considers. “I come from a family of doctors – my father and two brothers are all doctors, my sister’s a scientist – and there’s an element of people who dedicate their lives to helping others, or hoping to understand things, where there’s an innate gentleness. And on the other hand, they can quickly feel pretty superior! I was more interested in the gentleness.”

Isaac admits he was “pretty ignorant” about the genocide before working on The Promise. The Independent’s Robert Fisk has relentlessly fought to bring its well-documented events to public light, most memorably in the report recalled in his book The Great War for Civilisation (2005), when he and his photographer, searching for evidence of the mass killings in Margada, Syria, discover they are standing on a hill of skeletons. Mainstream cinema, though, has turned a blind eye. Micro-budget Armenian-language films apart, there’s been the fine Armenian-Canadian director Atom Egoyan’s modern-day meditation on the genocide, Ararat (2002), and maverick German-Turkish director Fatih Akin’s The Cut (2014), starring Tahir Rahim as an Armenian death-marched into the desert before a picaresque journey.