Writer/director Derek Cianfrance (“Blue Valentine”) didn’t write the original story for “I Know This Much Is True,” his new HBO miniseries about family burdens, sins, lineage, and legacy and more specifically about a brother trying to save his schizophrenic identical twin and himself in the process. But the heart aching, and compassionate series—adapted from Wally Lamb’s novel of the same name (read our review here), —is so chock full of Cianfrance’s trademark preoccupations of troubled family dynasties, the legacy of handed-down family traumas and damaged people struggling to survive emotional hardship, ‘IKTMIT’ feels like something the director would have willed into existence if Lamb hadn’t already written it. The material is so simpatico with Cianfrance’s humanist obsessions, the filmmaker seems to have been born to make it and fated to have crossed paths with the book.

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“Yeah, I felt the exact same way,” Cianfrance said of Lamb’s novel and the fated sense he felt when he read the resonant material (the first episode of which debuts on Sunday, May 10 on HBO). “It did feel like something I was born to make. It deals with all the same themes of family, of legacy, of paternity, and then even deeper themes I haven’t really explored is this idea of ancestry and heritage.”

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“I Know This Much Is True” centers on twin brothers Dominick and Thomas Birdsey (Mark Ruffalo stars in a stunning dual role), as Dominick tries to save his mentally ill brother. Following a gruesome self-inflicted incident, Thomas is transferred to a maximum-security mental health institution too dangerous for the meek Thomas.  Dominick makes it his mission to rescue his brother despite life being, by his admission, “a long 40 years,” thanks to his brother’s constant problems.

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Cianfrance, also known for the features “The Place Beyond The Pines” and “The Light Between Oceans” generally writes his own material or finds material that he wants to adapt. “I Know This Much Is True” however, came to him in a circuitous way.

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When once-optioned movie rights reverted back to Lamb, the author took matters into his own hands and approached Ruffalo’s agents, believing he was perfect for the role. Ruffalo read it, was immediately taken, and then the hunt to find a filmmaker was on. Ruffalo met Cianfrance at Sundance 2010 when both of them had films debuting there (Cianfrance’s “Blue Valentine” and Ruffalo’s directorial debut “Sympathy For Delicious”).

Their connection was “instant” and eventually, the plan was hatched to be co-conspirators on the project. “He felt like a brother to me,” Cianfrance said of their immediate bonding. “And I can’t say that’s necessarily unique to me because that’s kind of the quality that Mark has with anyone who meets him—he just feels familiar. Do you know what I mean? He feels like family to everyone because he’s just so damned human and empathetic and vulnerable and open. And he’s just a big, gigantic, beating heart.”

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That said, the idea of making a movie with twins and the challenges presented with an actor playing both parts wasn’t immediately appealing to Cianfrance, the filmmaker worried the technical challenges might distract from the emotional honesty of the piece. “Look, Mark, I’m really reluctant to do a technically tricky movie,” Cianfrance recalls telling Ruffalo originally. “There have been so many movies with twins, and many of them successful, but even ‘Dead Ringers,’ I’m trying to look where the seams are.”

The brothers were polar opposites: one ravaged by mental illness, the other suffering by proxy as the schizophrenic brother, and his various dramas plagued the family like a curse. Burdened with caring for his brother,  Dominick was angry, impetuous, and bitter and yet fiercely loyal. Thomas, sullen, hangdog-ish, and overweight from overmedication.

Eventually, a feasible plan was hatched that worked for both actor and director (fat suits were ruled out immediately). Ruffalo would lose weight (20lbs from his normal weight) and get lean and mean for the Dominick role. Production would stop for six weeks and Ruffalo would then put on 30lbs from his normal weight to capture the puffy and heavily medicated state of Thomas (the shoot was 16 weeks total).

The transformation was radical and immediate with the exact emotional effect that Cianfrance had intended for the characters. When Ruffalo was Dominick six weeks earlier, he was gregarious, palling around with the crew and willing to do 50 push-ups at the drop of a hat at Cianfrance’s request to put the character on the required volatile edge. Six weeks later, he could barely face the crew and wouldn’t come out of his trailer for an hour on the first day.

“I walked over to his trailer, and their Mark was as Thomas and it was so method and real,” he remembered. “He was terrified to come out. He was so ultra-sensitive. He was so vulnerable. For every way that Dominic was like a bear to wrestle with, this was like dealing with just like a fragile lamb.”

Empathetic, heart-wrenching and deeply emotional, “I Know This Much Is True” is also sprawling in scope and takes on dimension and textures of “The Godfather Part Two” as it explores the Birdsey family legacy and their Italian grandfather who emigrated to New York in the early 1900s and deal with issues of racism, class, xenophobia and more. This was also a major draw for the filmmaker.

“I’m of Italian American descent, and so this story presented an opportunity— and my Italian American heritage was always the part of me that inspired me to deal with legacy in the first place,” Cianfrance said. “It was all my heritage that was all twisted up in these stories that I was trying to unfurl as I was becoming a father myself, and inspired a lot of ‘Blue Valentine’ and a lot of ‘Place Beyond the Pines.’”

Thus, all of it was a perfect storm of material for Ciafrance to examine.

“Mark is insightful about human beings,” he said. “And that’s also what I’ve been trying to do is make movies about people, regular ordinary people, but cast them in extraordinary lights. Cast these ordinary circumstances and make these ordinary things feel epic. Because in my life, the smallest little family feuds or hurt feelings sometimes can feel very epic. And so that’s what I’d always been trying to do, is tell real, regular blue-collar American human stories that were open. That was specific but also open for the viewer to jump in.”

Much more from this interview soon. A painful, but beautiful story of family and not giving up on the ones you love, “I Know This Much Is True,” which also features a stellar supporting cast in Melissa Leo, Rosie O’Donnell, Archie Panjabi, Kathryn Hahn, Imogen Poots, and Juliette Lewis, debuts on HBO on Sunday, May 10 at 9pm ET.