The legendarily high-maintenance pop diva checked her glossy celebrity patina at the door to convincingly portray the film's Ms Weiss, a drab but deeply empathetic soul helping a troubled teenager in 1980s Harlem.
Far from the image Carey has cultivated for years, the character is no oil painting of music-video pulchritude, with her lank hair, a wardrobe of rayon sweater-coats and, yes, even a sparse moustache creeping across her upper lip.
"I had to lose all vanity," Carey said.
"I had to change my demeanour, my inside, layers of who I am, to become that woman."
How R&B's most unabashedly glamorous chanteuse came to sport facial hair - how Carey came to defy all expectations by delivering what some describe as an Oscar-worthy performance in Precious at all - is one of those quirky sagas upon which indie film-world dreams are made.
It turns out the alto with a five-octave range was not director-producer Lee Daniels' first choice.
He had considered Jane Fonda for the role and cast Carey only when Oscar-winning British actress Helen Mirren dropped out at the eleventh hour.
Daniels, who became chummy with the singer after casting her in his indie drama Tennessee, implemented Carey's deglamorisation process as a means to two ends - to ensure audiences would not be "taken out of the picture by seeing Mariah Carey", but also to antagonise the singer for her own good by making her look homely in the extreme.
"It wasn't just the director in me," Daniels explained, "but the big brother torturing his sister.
"This was just to irritate her.
At what point would she start screaming and run up out of this chair?"
To put a fine point on how unlikely all of this is, one need look no further than Carey's 2001 star vehicle Glitter.
A semi-autobiographical musical romance, the movie was trounced by critics, fizzled at the box office and netted the performer a Razzie Award for worst actress.
But since Precious premiered at last year's Sundance Film Festival, critics have been singing a different tune.
A reviewer for Variety called Carey's performance "pitch-perfect", while The New Yorker's Anthony Lane asks, "Hold on: a stern, song-free, compassionate piece of acting from Mariah Carey?
"It's for real."
Nonetheless, Carey finds her appearance in Precious painful.
"Hideosity!" she exclaimed, raising her hands in mock horror.
Carey was seated on the patio of the Polo Lounge in Beverly Hills, wearing a plunging black gown and impressively decked out in diamonds.
The divide between her luxury life and latest movie role could not have been more vividly illustrated, but Carey (40) left the disconnection unmentioned.
"I am glad people are telling me they don't recognise me," she continued.
"But when it comes to my scenes, I get like, 'Oh, I don't know if I can look'."
Precious is based on the acclaimed 1996 novel, Push, and Oprah Winfrey and Tyler Perry are executive producers.
It follows Claireece "Precious" Jones (newcomer Gabourey Sidibe), a 16-year-old Harlem girl whose hard-knock life provides a taxonomy of urban poverty's worst ills.
After enrolling in a literacy programme, Precious is reluctantly assigned to visit a social worker: Carey's Ms. Weiss.
"She still doesn't have a first name," the singer said with a laugh, popping a blini with a glinting mound of caviar into her mouth.
In her few but unforgettable scenes, the character plays a pivotal role in helping Precious pull out of her downward spiral.
Movie history is studded with A-list actors who subverted their prescribed images in a bid for greater respect and awards-season glory.
A few recent examples: Nicole Kidman's golden statue-grabbing turn (courtesy of a prosthetic schnoz) in The Hours and Charlize Theron's uglying up as a serial killer for her Oscar-winning portrayal in Monster.
Unlike some other performers, ego-driven careerism was hardly the deciding factor in Carey's transformation.
When Mirren, who previously worked with Daniels on his 2005 directorial debut, Shadowboxer, dropped out of Precious just days before she was scheduled to be on the set, the director had to scramble.
There was no time to audition replacement actresses.
Cue telephone call from Carey.
"Mariah calls me, 'Come over, dah-ling', I said, 'I'm not in the mood.
"I gotta cast this movie'," Daniels recalled.
"`What movie, dah-ling?"'
The two had forged what they describe as a brother-sister level of closeness working on Tennessee, a little-seen road drama Daniels produced that had a brief theatrical run this (northern) summer.
"I said, 'I'm doing Push'."
She had read the book.
"A light bulb went on over my head," Daniels said.
With Mirren's blessing, he hired Carey.
"I wanted to tap into what people don't see in her," Daniels said.
"I wanted to show the person I know she is when we're alone.
"One of the smartest, most intuitive women I've ever met."
A gay and opinionated aesthete, Daniels personally saw to uglying her up.
"We created Ms Weiss on the spot," he said.
"We started with the bags under the eyes.
Then I said, 'I'm going to put a moustache on you'.
Bound her boobs down, put this graymakeup on her, found this cheesy rayon material; put it on her.
I could see her hands shaking.
I was nervous for her, but I felt it was good for her to be out of her bubble."
The pop diva said she hopes to continue in independent films but has no plans to give up music.
But before leaving the topic of facial hair, she wanted to get something clear.
"Do I look like I have a moustache?" Carey asked.
She craned her neck forward and thrust her upper lip in a visitor's direction.
Nope, no 'tache.
"I'm not very hairy," Carey continued, displaying her forearm on which not a single hair could be seen.
"I've never had that issue."
