Behind his nuanced performance is the true story of a Connecticut family whose unimaginable loss is depicted in the film, which hits big screens nationwide Thursday.
Brenda and John Fareri's lives changed one night in 1995 when their daughter, 13-year-old Maria, complained of feeling achy, feverish and tired. There was talk of the flu. Their pediatrician told Brenda to keep an eye on her.
But when her symptoms began to interfere with her schoolwork – Maria was a newly minted eighth-grader with straight As – Brenda, a nurse, whisked her daughter straight to the hospital. "She had really deteriorated," she tells PEOPLE of that night.
Doctors were baffled. The chestnut-haired teen was in terrible pain and growing more feverish and fatigued as she lay in the hospital in Westchester County. Still, there was no diagnosis.
No one could possibly know that in Maria's sleep one recent night, a silver bat had sunk its teeth into her shoulder, leaving marks barely the size of a pinprick – tragically ensuring that no one would even notice the bite.
It was only when one telltale symptom put everything into horrifyingly sharp focus that answers finally came.
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By then, it was too late to save Maria. She had contracted rabies, which is fatal if not treated in time.
"The most difficult part was knowing there was no hope. Once you have symptoms, there's nothing they can do," Brenda says. "It was shocking. … I knew just enough to know there was not going to be a happy ending."
It was almost too much to bear when Maria died, and the couple searched for ways to manage their grief. The family – which includes adult triplets – endured the ordeal in a hospital not geared toward those tending to gravely ill children. Brenda wasn't allowed to get into bed with her dying daughter, and no private room existed for doctors to talk to the parents about the fatal diagnosis.
John Fareri (played by Duchovny in the movie), who "was a basket case" after Maria's death, kicked into gear. He and his wife raised millions of dollars and cleared endless bureaucratic hurdles to build a state-of-the-art pediatric hospital in Westchester, New York, to honor their daughter and help other parents in their position.
"You try to make sense out of something that doesn't make sense," John tells PEOPLE. "I'm a developer and builder and I couldn't do anything for Maria, but I channeled it into what I know."
The couple's dedication resulted in the Maria Fareri Children's Hospital at Westchester Medical Center, completed in 2004 and now ranked among the country's best for children. The Fareris say its atmosphere is uniquely happy: There's pet therapy, music therapy, poetry and an art neighborhood. All rooms are private, and all have space for family members to stay with their sick child.
"A lot of times this kind of grief breaks down a family," John says. "That's something no one tells you."
Duchovny, a dad himself, has praised the family's courage in making the film – and Brenda believes he did their story justice. "He really became John. It's amazing. He's a phenomenal actor," Brenda says. Equally important to the couple: "He's also become a friend."
Watch the Louder than Words trailer below.