Once upon a time, a young ingenue took a trip across Los Angeles to meet Hollywood’s equivalent of the big bad wolf. The controversial Danish director Nicolas Winding Refn was casting the lead role in The Neon Demon, a nightmarish fable of the fashion industry, and feared that wholesome Elle Fanning might not be tough enough to stay the course. So Refn asked her out of the blue if she thought she was beautiful – a question deliberately intended to trip her up and make her squirm. The tactic succeeded. But only up to a point.

“In the end I said yes,” Fanning recalls. “I said that yes, I did think I was beautiful. Because I knew it was a test, he was trying to get me to crack. And I thought that this was what I should say, what the character would say. And it was right, it worked, because I got the part.”

It was the correct answer for Jesse, the pristine heroine of The Neon Demon, who wafts her way through an LA inferno of vampiric runway models. But maybe it says something about 19-year-old Fanning as well, who has spent a lifetime in Hollywood and managed to emerge similarly unsullied. “It’s not that I don’t think terrible things can happen to young actors in Hollywood,” she insists. “I know they definitely do. I’ve just been very fortunate that they haven’t happened to me.”

We meet inside a beachfront hotel during the Cannes film festival, where sunlight bounces off the marble walls and motorists toot their horns in the street down below. Fanning, it transpires, has been in town all week, attending a variety of events in a variety of garments. Her array of stylists refer to themselves as “team unicorn”, as though they are administering to the needs of some mythic beast. Today, they have put her in a sheer dress of such perfect whiteness that one quite fears for its safety; a breath of air might irreparably stain it. “Don’t worry,” a publicist murmurs in the actor’s ear. “We can get you changed before you go to lunch.”

Fanning landed her first acting job at the age of two and has been working steadily ever since. She points out that her latest film, The Beguiled, is actually her first role as an adult; the first in which she was not required to have her mum accompany her on the set. “So that makes it a very special film for me,” she says. “It was a taste of freedom. Figuring it out. Growing up.” She gestures down at her 5ft 10in frame. “Emotionally, not physically. I’m tall enough as it is.”

In The Beguiled, Fanning has even graduated from playing the innocent to playing the wolf. Sofia Coppola’s film is a juicy southern melodrama, draped in spanish moss and thrumming with sexual tension. Nicole Kidman stars as the upright headmistress of a Virginia girls’ seminary; Colin Farrell as the wounded civil war Union soldier who initially thinks he has found heaven. Fanning, as befits her newly adult status, lends firecracker support as Alicia, the oldest of the girls, who steals away from the dinner table to plant a kiss on the soldier’s mouth. “We can show him some real southern hospitality,” she coos to her friends, shortly before the film pitches towards outright bloody horror.

Watch the trailer for The Beguiled

Fanning loved making The Beguiled, and her enthusiasm is infectious. She talks in a torrent, like a river in spate, merrily bursting the banks of each individual question to the point where I am tempted to discard them altogether. She says she relished playing “the bad girl”, although is Alicia really so bad? She is just bored and hormonal; anyone that age might have behaved the same way in her position. Fanning adds that she loved acting alongside Kidman, who has been her idol for years; Moulin Rouge is one of her all-time favourite films. “Also, she’s tall. And I’m tall,” she says. “Tall actresses are few and far between. There’s Uma Thurman, Nicole Kidman, I think that’s about it, so we’ve got to stick together.”

Most of all, it seems, she loved working with Coppola, who as writer-director conspires to put a shrewd feminist spin on Don Siegel’s original 1971 picture. Fanning points out that she last collaborated with Coppola on the refined, minor-key Somewhere back in 2010, when she played the sweet, soulful daughter to Stephen Dorff’s car-crash Hollywood actor. So this film was like coming full circle; an amazing experience, she gushes. “Sofia has this way of working that, like, comes right from her soul,” she says. “And it’s a very elegant, graceful way of working and she’s very respected and she’s had all this great success, but what I admire is that she’s done it very tastefully, with grace and talent and creativity. She’s really created a stamp and you can’t recreate it. And this film definitely has her stamp, but it’s different, too, because there’s blood in this movie, which is crazy, right? For her, right, it’s crazy.” She gulps a breath and collects herself. She says: “But she does it with elegance.”

Fanning was born in smalltown Georgia, the child of southern Baptists, but took off for the west coast when she was barely out of nappies. In a sense, she explains, the family was simply riding on the coattails of her older sister, Dakota, who was already securing roles alongside Sean Penn and Reese Witherspoon and has since gone on to forge a successful career of her own. You can see her in Twilight, War of the Worlds and Kelly Reichardt’s excellent Night Moves.

The way Fanning Jr tells it, she and Dakota were always playing make-believe in their bedroom. Acting was merely an extension of that. “Even when I was really young, I always knew it was acting. I mean, I did a movie called The Door in the Floor with Jeff Bridges when I was four and I always knew it was just pretend – that Jeff Bridges wasn’t really my dad. And yeah, I know it’s a job, I know that it’s serious. But it’s also about dressing up and pretending.”

It’s an outlook that appears to have served her well. On screen, right from the start, Fanning has been a spookily nuanced performer, possessed of a quiet intensity that rather belies her off-screen burblings. She played a stoic adventurer in JJ Abrams’s Super 8 and rustled up a heart-piercing performance as an angst-ridden British teen in the acclaimed Ginger and Rosa. I loved her as Jesse, the aspiring young model in Refn’s sugar-frosted horror show, but I also loved her as the chaste best friend in Mike Mills’s 20th Century Women. Fanning grew up in public and she has made it look painless – although she is quick to insist this was not always the case.

Fanning with Ryan Lee and Gabriel Basso in 2011’s Super 8. Photograph: Allstar/Paramount Pictures

“When I was 12, I grew seven inches in a year. And it hurt every day. On Somewhere, I grew two shoe sizes during filming. And I would pass out a lot because I hadn’t grown into my body. So that wasn’t a great time. It definitely wasn’t painless.”

If she counts that as pain, I reckon she got off very lightly. I am not even sure I quite believe her. Isn’t the life of a Hollywood child star meant to be a DayGlo gothic nightmare, a kind of prolonged dress rehearsal for Whatever Happened to Baby Jane? I want to hear about the shrieking brats at the open-call castings; the grasping parents who plunder their kids’ bank accounts. I have read that Fanning’s mother threw in her job, moved the family to LA. Please tell me she has been pushy and controlling at times?

Fanning, though, is having none of it. “Mothers in the industry get a really bad name,” she says. “Which is so not fair, because my mother is amazing and she sacrificed so much with her life out in Georgia, which she gave up to do this beautiful thing which is following a child’s dream. So I hate that view. I hate that idea. Without my mother, I wouldn’t be here, I wouldn’t even know what I was doing, maybe. I had the luxury of knowing what I want to do because of her.”

It strikes me that the logic of this last statement risks undermining her argument, but never mind. Time is running short and Fanning is back on a roll; she is the river in spate. She tells me that she comes from a family of athletes. Her mum played tennis, her dad played baseball and her maternal grandfather was a quarterback for the Philadelphia Eagles.

“I have the sports drive,” she says. “It’s in my blood; I can’t help it. Actors and athletes are similar, I think. You set a goal for yourself; you’re ready for a challenge. You prepare for the match or you’re getting ready for the role. The adrenaline when you have a big scene to do is huge. I felt that way when I was making Somewhere. I felt that way when I did Ginger and Rosa, when I was 13 – that’s maybe the role that signified the most for me, because I grew as an actress on that film and I felt much older at the end of it. I had a British accent in it. I dyed my hair red, when I had always been blonde, and so I didn’t feel like myself at all. Something clicked. I thought: ‘Oh, this is what acting is supposed to feel like.’”

Her southern heritage is important to her, but these days she suspects she has left it far behind. Fanning’s grandmother still lives with the family and is “a complete southern belle, very thick accent”. Her parents, too, still sometimes pine for home. She shakes her head. “But LA is my home; I’m a California girl.” Having headed west to seek her fortune, she has now staked her claim. The present is rosy and the future looks golden.

Fanning explains that the initial move to Hollywood was made in the spirit of adventure. It was a fun family diversion; not meant to last. “My sister says that it’s like we’re still on this trip to LA and that one day soon we’ll all go back to Georgia. But it’s been, like, 16 years now. And I say to her: ‘Look, this is us, this is our life.’ I say to her: ‘Look, we are never going back.’”

The Beguiled is out 14 July.