Introverted and awkward or diva-like and demanding, musicians can be a tricky bunch.

They're far more volatile and less media-trained than your average Hollywood star. Approaching an encounter with Macy Gray, you can't help but ponder where she will fit in. The Grammy-winning R&B singer doesn't have the best of reputations; she famously stormed out of a Vogue photo-shoot where she was being snapped by Lord Snowdon. He called her "odious", noting "in 55 years in this business, I've never met such a rude woman".

Whatever the truth, she has an eccentric streak. In her garage stands a bronze statue of herself – naked. She appeared on the US show Dancing With The Stars, only to be the first voted off amid rumours she'd quit several practice sessions out of frustration. She even managed to commit the cardinal sin of forgetting the lyrics to the American national anthem when she was invited to sing at a football tournament in her hometown. Failure, it seems, has defined her as much as success.

"I think that anybody who is a true artist - you only do it because you love it, because there's no guarantee," she says, sitting in the ballroom of the Martinez Hotel in Cannes, France. "You have to have some kind of passion or deep love for what you do. There's no other reason to do it. It's not like you're going to get a salary every week."

Today, Gray is beaming; there's no sign of the diva who refuses to look interviewers in the eye. Dressed in a pinstriped trouser suit – the stripes a vivid red, coursing down the charcoal-coloured material like veins – she arrives for our interview barefoot. Standing at 6ft tall, everything about her is larger than life. Her ears are weighed down by a pair of chunky earrings and her eyes adorned by huge false eyelashes. But it's her hair – a crazy afro-shock of amber, making her look like the lead character in David Lynch's Eraserhead – that leaves the most lasting impression.

At least until she opens her mouth, delivering her words in that unforgettable Minnie Mouse squeak of hers. "Everybody always laughs at me when I talk," she sighs. If that's true, they certainly don't when she sings, emoting in that soul-stirring rasp. Since her 1999 quadruple platinum-selling debut album On How Life Is, she's sold more than 15 million records worldwide. She's also been nominated for five Grammy awards, winning one for Best Female Vocal Pop Performance on mega-hit I Try, which came from that debut record.

We meet the day after Gray has strolled down the red carpet at the Cannes Film Festival with such luminous talents as Nicole Kidman, John Cusack and Matthew McConaughey. She's still floating on air.

Gray is here with The Paperboy, the third film by Lee Daniels, the exotic talent behind the Oscar dark horse Precious from three years ago. While that film was a searing study of an illiterate, obese teenager, The Paperboy couldn't be more different. Adapted from the novel by Pete Dexter, it's a sexy, sultry, sweaty slice of Southern noir, a melting pot of sexual and racial tensions. Set in Florida in 1969, the story revolves around a big-shot reporter named Ward Jansen (McConaughey) who arrives back in his hometown to investigate the innocence of a Death Row inmate (Cusack), convicted of killing a racist sheriff.

Thrown into the mix is Kidman's femme fatale Charlotte Bless, who has a penchant for men behind bars, and Ward's younger brother Jack (Zac Efron), who is soon panting after Charlotte. Then there's Gray's character, Anita Chester, the maid to Ward and Jack's folks. She's also the film's narrator, our eyes into this world.

"Lee called and he was so excited – he said, 'You're going to be the narrator,'" Gray recalls. "And I thought, 'But my voice is funny! Do you think that's going to work out?' He goes, 'Are you crazy?' So I did it, but I didn't know until yesterday that it made sense."

To prepare, Gray had the perfect research tool. "I have a housekeeper so I watched her a lot," she explains. "I discovered a lot about her, which helped me. I noticed she's always irritated. Ever since she's worked for me, she's always p***** off. I realised she doesn't want to mop my floor and she doesn't want to wash my socks. So I brought that to Anita. She always has a little bit of an attitude. I learned a lot, watching how she comes to this revelation and learns a lot about life and her place in this family, and how she won't ever be fully accepted by a family that she wants so bad."

Unquestionably, in a film where sleaze and sex dominate, Gray provides the emotional ballast. Born in 1967, two years before the film is set, she may not have lived through this era of racial inequality – but she was able to talk to her mother about it. "She told me the vibe of being black then. You get insults and are always made to feel inferior. I always wondered, 'How did you deal with that?' She just told me you don't ever become numb to it, where it doesn't matter – even today she'll feel it. But you get to the point where you get tired of fighting against it, so you start fighting for yourself and you have to shut all that out."

Gray is no newbie when it comes to acting. She started in 2001, in Training Day, opposite Denzel Washington. Roles have come since in Prohibition-era musical Idlewild, Tony Scott's bounty hunter movie Domino and the garish reboot of Jules Verne's Around The World In 80 Days – but Gray doesn't consider it "a jump" from music. "A lot of times, you do one thing and people think that's all you're going to do, then when you do something else, they get so shocked. But it's natural for people to do a lot of things – it's part of life."

Gray also appeared in Daniels' first film, the 2005 thriller Shadowboxer, in which Helen Mirren and Cuba Gooding Jr played contract killers who just so happen to be stepmother and stepson, and lovers. Gray, who played one of their victims, admits she was taken with Daniels even then. "Lee wasn't Lee at that time," she says. "He was still his crazy self but he hadn't done Precious, so a lot of people didn't take him very seriously. He called my agent, and it went back and forth – 'I don't know if you should do this movie, this dude is crazy, he doesn't have any money'. They kept trying to talk me out of it."

She admits it was Daniels' energy that drew her to take a risk on Shadowboxer. "I was always so intrigued by how nuts he is, and how it feels that he's not afraid of anything. He's not crazy in a bad way, but he will do anything – he's not afraid to try things or to mix them up or to make things different."

Gray had to fight off interest from Oprah Winfrey for the part. What's the director's take on Gray? "She's nuts, in the nicest possible way," he says. "But she's brilliant, a wonderful actor. She gives me her spirit."

Where that spirit came from is a curiosity. Gray was born and raised in Canton, Ohio – "a real laidback town" where little happened. "I definitely wanted to get out of there," she says. Her mother, Laura, who worked as a maths teacher, split from her father when Gray was a baby. When Gray was six, her mother remarried, to steelworker Richard McIntyre.

She grew up as Natalie McIntyre, though it didn't take long for her to find her stage name. When she was six, so the story goes, she fell off her bike and found herself lying beneath a mailbox bearing the legend "Macy Gray". The name belonged to an old man in the neighbourhood who later tried to sue her when she got famous. She began writing stories about Macy, long before she adopted the name. She learned piano too, from the age of seven – a skill her family encouraged, though her parents were "not the sort to tell their children how great they were".

As a youngster, Gray attended an elite prep school, Western Reserve Academy in Hudson, Ohio – along with a bunch of "racist white kids", as she bluntly puts it. She was not always a good student. "I would cut class a lot, but I was real smart so I could always pass." Forever in detention, it was during one of these sessions that a teacher put his hand on her leg. Bringing a sexual harassment charge against him, a couple of months later she was expelled. "I was told my grades weren't good enough. My grades were fine but that's the excuse they gave."

A misfit she may have been, but Gray found her groove when she left Canton for Los Angeles, enrolling in a scriptwriting course. "That's when you get into little circles where it's cool to be weird," she says. Falling in with aspiring filmmakers and musicians, she began to write lyrics – initially, she says, so she could hang around with a guitarist she fancied. When a singer failed to show for a demo session, she filled in; the tapes began to circulate and the leader of a local jazz band asked her to join his outfit.

Gray was destined for more than lounge singing, however. Diving into the Los Angeles underground music scene, she worked as a hostess at a late-night club, coming into contact with such pioneering bands as The Roots and English musician Tricky. Her own chance came when she signed to Atlantic Records in 1994 – although it was almost over before it began, Gray completing a debut album that wouldn't see the light of day after the label got unnerved by her unusual vocal style.

It didn't help that her personal life was imploding. She split up with her husband, with whom she had two children, while pregnant with a third child, just as she received the elbow from Atlantic. With no home to speak of, she slunk back to Ohio – pregnant – to live with her mother. "You know, no grown-up wants to move back into their mom's," she says. "You feel such a loser. But where else was I going to go?"

Fortunately, she had a supporter in the shape of publishing executive Jeff Blue, who – after months of badgering her to meet him – finally got his wish. Having been persuaded to pour the heartache of the previous four years into new material, she went on to record her (second) debut, which Blue touted around the industry under the pseudonym Mushroom. Signed to Epic, Gray released On How Life Is, a fusion of Sly And The Family Stone, 1980s-era Prince and contemporary hip-hop. She followed it with another million-selling album The Id, released days after the 9/11 attacks of 2001.

If those early days were flush with success, Gray hasn't had it all her own way. Her third album, The Trouble With Being Myself (released in 2003) was deemed a flop. Her behaviour began to spiral out of control too (she boasted in one interview of stealing a painting from a hotel room). The inevitable downside of non-stop partying was also taking its toll. "I'd been indulging in excess of everything and it started catching up with me," she says. "I wasn't focused on my craft any more and I didn't nurture it. I have this talent but I didn't take care of it."

By the time of her comeback album Big four years later, she was anything but. Her fanbase was disappearing and she'd switched labels and assistants; even her children changed schools. "You're supposed to learn something," she shrugs, though it was a period of misdirection and misrule. Wisely, Gray set out on her own to release her fifth album, The Sellout. "It's about all the mountains I've been climbing to be where I want to be in my personal and professional life," she announced on her website. If anything, it was an apologetic love letter to her fans, an attempt to win them back.

Since filming The Paperboy, Gray has been touring Europe and the US, promoting her new material, which has seen her pay tribute to all manner of artists with the release of two covers albums. Covered saw her take on Radiohead's Creep, Nothing Else Matters by Metallica and Eurythmics' Here Comes The Rain Again among others, while Talking Book was a track-by-track reprise of Stevie Wonder's album on its 40th anniversary. Somehow, she's even found time to shoot another indie movie, Little Lake, for first-time filmmaker Jasmin Sharon, playing a "hippie psychic" who helps a young girl's coming-of-age.

At 45, Gray is no longer a party animal. The drug taking days are behind her. Diagnosed as bipolar, she lives with her three teenage children and her sister. Honesty defines her approach to parenthood.

"You do have to tell your children the truth because either they get in the habit of lying or they grow up and they don't trust you." She smiles as she tells an anecdote about being in a restaurant with daughter Aanisah when her Paperboy co-star Zac Efron was also there. "I said, 'Is that Zac? No way!' My daughter said, 'He's so beautiful!'" You could say the same about Macy Gray.

The Paperboy (15) is in cinemas on March 15.