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Before it was even published in the summer of 1998, Hollywood began circling “I Know This Much Is True.” Despite its Tolstoy-esque length and often grim subject matter, Wally Lamb’s novel about identical twin brothers became a bestseller, boosted by what was then the ultimate endorsement: Oprah Winfrey selected it for her book club.

Twentieth Century Fox gobbled up the movie rights for a reported seven-figure sum. Big-name talent including Matt Damon, Jonathan Demme and Jim Sheridan were attached to the project along the way, and numerous screenwriters attempted to tame the 900-plus-page book, which spans seven decades. But the adaptation languished in development hell.

“I had written an unreasonably long story and nobody could quite figure out how to fit something that large into a two-hour box,” Lamb said. “I thought, OK, this is just what happens when Hollywood starts flirting with you, like the cute girl who says, ‘I’m going to go to the prom with you’ but it never happens.”

Lamb can finally buy that corsage: The adaptation of “I Know This Much Is True” has arrived on HBO, thanks in large part to star and executive producer Mark Ruffalo.

Written and directed by Derek Cianfrance, the six-part series charts the saga of Dominick and Thomas Birdsey (both played by Ruffalo), who are raised in a tense and sometimes violent household. Thomas is a paranoid schizophrenic in and out of institutions most of his adult life, while Dominick is a house painter who is both utterly devoted to and resentful of his brother, at the cost of other relationships — including his marriage.

After 15 years and many false starts, the rights to the novel reverted to Lamb. When his agent asked him who he’d like to play the twins, he immediately thought of Ruffalo, an actor he’d admired since “You Can Count on Me.”

The agent reached out to Ruffalo, who quickly realized it was something he wanted to pursue. “It was just beautiful and it was personal and it was really challenging and all these things that ticked the boxes for me,” he said.

Before he even finished the book, Ruffalo fired off a note that Lamb describes as “the most incredible e-mail I’ve ever received.” “This will happen,” the e-mail promised. “I can all but guarantee it.”

Ruffalo quickly decided that the book was better suited to an episodic television series than a feature film.

“It’s not an accident that there’s 22 drafts of this and they couldn’t figure out how to do it,” he said. “And we’re in this beautiful golden age of television where we can novelize a TV show.”

Even by the standards of premium cable, “I Know This Much Is True” is a difficult watch, portraying mental illness, racism, grief, domestic violence, rape, child abuse and infant death. “It was like eating a bowl of grief,” said Rosie O’Donnell, who plays a social worker.

Whether such an emotionally taxing story will appeal to viewers in an already dark period is a gamble, but the show’s creators believe its themes of loss, family and redemption are universal.

“What we were hoping to do with the whole film was to open this experience up to everyone,” Cianfrance said.