HBO’s new series “I Know This Much Is True” has the word "true" right there in the title, so it’s easy to see why viewers are wondering if the show is real. The Mark Ruffalo series tells the seemingly realistic story of twin brothers, one of whom suffers from severe mental illness, but believable doesn’t always mean real.

“I Know This Much Is True” is a fictional story. The HBO limited series is not based on a true story, but it is an adaptation of the 1998 Wally Lamb novel of the same title. The bestseller was part of Oprah’s Book Club and hit the top of the New York Times Bestseller list.

It took over 20 years, but the hit novel is finally being made into a screen adaptation. HBO will tell the story over six episodes, in which Ruffalo pulls double-duty playing both twin brothers.

The “I Know This Much Is True” TV series keeps the book set in the 1990s, with both Dominick and Thomas Birdsey approaching middle age and Dominick flashing back to his childhood. Thomas suffers from paranoid schizophrenia, and Dominick does his best to love and protect his brother. However, he has his own inner turmoil that goes far beyond the stress of his brother’s mental illness. HBO calls it “an epic story of betrayal, sacrifice and forgiveness.”

Though he is best known as The Hulk/Bruce Banner in Marvel’s “Avengers” films, Ruffalo has been talking about mental illness for quite some time. Previously, he starred as a bipolar man in “Infinitely Polar Bear” and has been open about his own experiences with depression and ADHD.

READ: Mark Ruffalo Spoiled ‘Avengers’ Movie Nearly A Year Before Release

Ruffalo says the filmmakers were careful with how they portrayed paranoid schizophrenia. “There’s a certain behavior of dealing with schizophrenia. It’s the voices. It’s the paranoia. It’s the auditory and visual hallucinations. But schizophrenia isn’t a personality trait,” Ruffalo told Fast Company. “Capturing Thomas’s duality to Dominick was really important. His sweetness, his gentleness, his femininity, his softness. There’s a mythological story being told here that’s couched in this kitchen-sink drama.”

“I Know This Much Is True” premieres Sunday, May 10 at 9 p.m. EDT on HBO.

I Know This Much Is True
Mark Ruffalo plays two characters in HBO's limited series "I Know This Much Is True." HBO