ET was on set during the filming of the harrowing series, and spoke with the show's stars about the nuances of their dark characters and how they got inside of the minds of the real-life people they portray.
The drama series, which is based on a true crime story of murder and abuse, follows Dee Dee Blanchard (Arquette), a woman with Munchausen syndrome by proxy, a mental illness that lead to her making her young daughter, Gypsy Rose (King), purposely sick to gain attention and sympathy from others.
"For years [she] kept Gypsy prisoner in a wheelchair, you know, had her have operations she didn't need, she had a feeding tube [even though] she could swallow fine," King said of her character. "It was just so many years of lies and imprisonment."
According to Arquette, she approached the character of Dee Dee not as if she were playing someone who was disturbed, but instead as if she truly had the best intentions for her daughter.
"Dee Dee believes all of this is necessary," Arquette explained. "She kind of thinks that she's smarter than the doctors so she really does think that her daughter does need these things."
Throughout the years of abuse, Dee Dee told friends, family and even medical professionals that Gypsy was mentally disabled, younger than she really was, that she had numerous degenerative illnesses and even that she had cancer.
As part of the depiction of her Munchausen syndrome by proxy, they shot a scene in which Arquette shaves King's head bald, which was just one big part of the 19-year-old actress' major physical transformation that made her nearly unrecognizable.
"I shaved my head, [and] we have Gypsy's glasses, or a very good replica," King explained. "I wear fake teeth. There's four stages of fake teeth!"
In The Act -- which was greatly informed and influenced by the HBO documentary Mommy Dead and Dearest, which details the brutal facts about the horrifying case, and features an interview with Gypsy, who is serving a 10-year sentence after pleading guilty to second-degree murder.
After years of suffering at the hands of her mother, Gypsy secretly began going online where she met a man named Nicholas Godejohn (played in The Act by Calum Worthy). The pair struck up a secret on-line relationship and she began to confide in him about the lies and trauma being inflicted upon her, at which point they concocted a plan to kill Dee Dee, which they ultimately went through with -- and the details of the story got even weirder as it unfolded.
"Her family flushed her own ashes down the toilet," Arquette recalled in disbelief. "I mean, that's very intense."
The story only gets more intense, and viewers will get an inside look at the madness, the murder and the mystery of Hulu's The Act, which premiered on Wednesday.