Dakota Fanning, who has been working for 16 years and is still only 22, remembers the birthdays of her childhood not by the parties or the presents, but according to the movie sets she was on at the time. The year she turned seven, for example, she was filming I Am Sam, her first movie, in which she appeared with Sean Penn, whom her mother assured her was a “great dramatic actor”. (“I was like, OK!”). At 10, she was in War Of The Worlds with Tom Cruise. And at 15 she was in her first semi-adult role in The Runaways, a biopic of the 1970s band of the same name and memorable, to Fanning, because it fell within those years when everyone kept asking if she was “worried you’re going to go into a downward spiral?” To which, she says, the only possible response was, “What are you talking about?”

We are in a studio in New York, where Fanning is eating a late lunch after the photoshoot. She’s dressed in an oversized beige jacket and has overblown hair and makeup, so that, she says with an eye-roll that seems like a last vestige of adolescence, “I look crazy.” In fact, what she finally looks like is someone who’s age has caught up with the life she is living. Fanning lives on her own in a rented apartment and is a student at New York University, breaking off from her studies every few months to make a new film – most recently, an adaptation of Philip Roth’s American Pastoral, co-starring and directed by Ewan McGregor, and Brimstone, a 19th-century thriller co-starring Guy Pearce.

Minidress, embroidered overskirt and lace shirt, all by Delpozo. Rings by Ara and Alison Lou. Opening photograph: Dress by Dior. Earrings by Maria Tash. Photograph: Erik Madigan Heck/The Guardian

At 22, Fanning’s manner and eloquence, which as a child were always described as almost spookily mature, no longer stand out. Actually, she says, it was never as straightforward as the freak show, adult-in-a-child’s body descriptions of the time, but more “a balance of feeling like the oldest person in the world and still very young”. Fanning has about her what Penelope Fitzgerald, in her novel about child actors, At Freddie’s, described as “the bright airs of the indulged”, a simultaneous innocence and self-conscious performance of innocence. If she behaved like a 40-year-old professional when she was eight, when Fanning now says, “I’ve had so many experiences and been so many places in my life, and met so many people, and I’m still so young and have three more lifetimes!” she sounds very young indeed.

What she is, without doubt, is a pro, who according to those who have worked with her is polite, industrious and ferociously disciplined. Fanning has been working for so long that most of the technical requirements of acting are imprinted on her as muscle memory. “I know how to hit a mark without looking. I instinctively know where my eye line should be. That’s all 100%. But your character and the story are always different, so the emotional part is not muscle memory. You’re still surprised by stuff and get the adrenaline.”

With Tom Cruise in War Of The Worlds, 2005 Photograph: Everett/REX/Shutterstock

Off set, Fanning gives the impression of being almost aggressively good-natured. Her relationship with fashion is, as with so many things in her life, extremely sensible. She tells me about the outlandish latex dress she owns, for “special occasions”, but she also likes the ease of an unfussy wardrobe. “Part of me wants a uniform, and part of me loves the spontaneity. I’m this weird balance of two complete opposites.” She’s also a pragmatist. “I’m a shorter person, so I’ll see something and think, ‘That looks amazing!’ But that person is 6ft 1in. It’s going to look different on my 5ft 5in frame.”

This may sound trivial, but for a woman in Fanning’s line of business, it is no small thing to be at ease with the looks, height and shape you were born with. From the outset, she says, the horse race aspect of being a female Hollywood star was something that she was very good at reassuring herself didn’t matter. This was true even at the age of five, when her mother, at the behest of a local drama teacher in their native Georgia, took Fanning to an agent in Atlanta for an audition. This was not a case of Mama Rose and Baby June. Fanning’s parents (her father is a salesman, her mother a retired tennis player) were, she says, simply acting on their daughter’s enthusiasm. The teacher taught Fanning at a summer camp and “thought I could do commercials or something”. Sure enough, after being signed up by the agent, Fanning did three commercials in a row.

They would always pick the girl with the really long hair. I had short hair and was different looking

She also began to get knocked back a fair amount and here, perhaps, her response was unusual. “I was always able to decipher what mattered from what didn’t,” she says. “I would go on so many commercial auditions and I would hardly ever get them, because they would always pick the girl with the really long hair. I had short hair and was different looking. And I would say, ‘You know, they probably just really wanted someone with brown hair, or longer hair.’ And my mum would say, ‘You’re probably right.’”

During her early years of fame, it was often reported that Fanning’s parents, at the first whiff of success, uprooted their family and moved to LA for “pilot season”, hoping to get their daughter on a TV show. In fact, her mother took her for a six-week visit to her aunt in LA “so we had a place to stay and a person to visit”, and let her attend auditions while they were there. Fanning kept getting work, and her mother kept pushing back their flight home; this went on until she was cast in I Am Sam a year later.

“My parents always thought this was temporary, and always made it very clear that if I wanted to be done, that was fine. My mum would say, ‘Oh, we’re from Georgia, we go back and forth.’ Meanwhile, we hadn’t been back for, like, three years. She couldn’t come to terms with this new life.” Eventually, Fanning’s father, grandmother and younger sister Elle (now a very successful actor in her own right: she recently starred in Nicolas Winding Refn’s The Neon Demon) joined them in LA and the new life began.

With Sean Penn in her debut movie, 2001’s I Am Sam. Photograph: Lorey Sebastian/AP

The unsavoury aspect of child actors holding down adult jobs, with adult pressures and responsibilities, is one that Fanning seems, blithely, to have sailed through unscathed. One advantage of being so young when she started is that there was almost no sense of before and after, no dislocation. “It was fine. My mum was with me. I was visiting my cool aunt. And I was enjoying what I was doing.” She was too young to know who any of the famous actors she was working with were; the only time she freaked out at a celebrity was when she saw Britney Spears in a shop, because “she was like God to me”.

When you’re on set, you’re miserable, it’s cold, you’re tired. And yet there’s no place I’d rather be

A year to the day after she arrived in LA, she was cast in I Am Sam, the story of a man with learning disabilities (Penn) fighting for custody of his daughter, with Michelle Pfeiffer in the role of his lawyer. Although the movie wasn’t a hit, Fanning was nominated for a Screen Actors Guild award and Penn for an Oscar. It was a short hop from there to Fanning being cast by Steven Spielberg in the sci-fi mini-series Taken, then as the younger version of Reese Witherspoon in Sweet Home Alabama and, within another year, to starring in movies opposite Robert de Niro and Denzel Washington. This could all have ended in Macaulay Culkin-type scenarios, but for some reason “it didn’t go the gross way”, in spite of the fact that for five solid years after she turned 13 it is all anyone asked her about. “This is why people go into a downward spiral, because they have people making them feel like they have something to be insecure about. I couldn’t stop from growing, or changing. I also couldn’t stop enjoying what I did.” What had once been fun, changed overnight: “Now there was expectation – to fail, to succeed, to come through to the other side.”

She puts her ability to endure this kind of pressure down to the way she was raised, and specifically to her mother. “My parents, even before I started doing this, were the kind of parents who expected a certain behaviour and focus in life. Manners were important and so was respect for adults: that was taught to me from birth. And that became a sense of professionalism, as I started to understand the decorum of movie sets: have respect for the people who are trying to help you.” It was also a question of mimicry. “When you’re a kid, you are so impressionable. And you’re soaking up the focus of the set.”

Dress and shoes, both by Dolce & Gabbana. Earring: Nikos Koulis. Photograph: Erik Madigan Heck/The Guardian

Fanning was home-schooled until she was 13, and during those early years most of the people she hung out with were adults. She retains a slight formality of speech that is either a stylistic quirk or, as it occasionally seems, a hangover from the years she spent playing up to adult expectations. “Someone once said about me that I talk to everyone the same, no matter what age they are,” she says. “I don’t see kids and adults. I see everyone the same.” This statement makes me wonder how much fun she actually had in her childhood, but Fanning is having none of it. “I loved it. I think it was such a special way to grow up.” She loved the acting so much, she says, that in her teens she never went through a period of having to decide whether or not to keep going with it.

“I do realise there are alternatives, but I also don’t at the same time,” she says. “Because it’s become a given that this is what I want to do; whether I’m still allowed to do it is up to a lot of other people. Often when you’re on a movie set, you’re miserable, it’s cold, you’re hungry, you’re tired, it’s still dark out. And yet there’s no place I’d rather be. It’s the happiest I am, and the most calm.”

She doesn't give her actor sister Elle advice: 'Who am I to tell anybody what to do?'

I ask if, during those formative years when she was making so many movies, she was ever in danger of having too little left over to form a stable identity. “No, because I’m not the kind of actor who stays in character. I go in and out all day, every day.” It helped that, after she went to a regular high school at 13, she had a good, solid group of friends, many of whom wound up in New York and form her social circle today.

Still, it can be lonely. One of the things she finds strange is how “I have all these weird little isolated lives on film sets. Since I turned 18, I go everywhere alone, so I think no one will ever know how it was to live and film 35 minutes outside of Philadelphia in a small town, where I was totally by myself. No one will know what my little life was like.”

In Charlotte’s Web, 2006. Photograph: Moviestore/REX/Shutterstock

When Fanning moved to New York, she went through the universal rite of passage of waking up one morning in her apartment and realising that, without her mother there, “there were no clean towels. Oh my God, I have to find time to go wash the towels.” Her mother stayed in LA, carrying on her duties as on-set chaperone to Fanning’s sister, Elle, who is four years her junior and upon whom, somewhat surprisingly, Fanning hasn’t pressed a lot of advice. “Because, on the one hand, of course I have all this experience, but it’s my experience and it won’t be true for anyone else. Who am I to tell anybody what to do?”

This attitude of almost studious blandness has clearly served Fanning well and provided some kind of antidote to the years of exposure. At NYU, she is studying “the portrayal of women in film”, something she says with a smile to acknowledge both that she really can’t leave the movie world alone, and that she is in an odd position of writing essays on a culture to which she herself contributes. The decision to go to university was a pragmatic one made on the basis that too many actors become depressed, aimless or lost in the downtime between movies. “You have a concentrated amount of time that’s really busy and then you have a lot of freedom. So what are you going to do?”

With her sister Elle. Photograph: Matt Baron/BEI/Shutterstock

At NYU, particularly the Gallatin School where she is a student, nobody cares about fame and Fanning’s professional absences are seamlessly accommodated. She just has to turn in her essays, which is fine, because she is very used to doing academic work on set. But the most tongue-tied she gets – apart from when we talk about money – is about her thesis, the subject of which puts her in the tricky position of criticising the movie business, something Fanning is either too loyal, shrewd or well-mannered to do. And so, she says, while her experience informs some of her essays, she does not bring her critique of women in film to the set.

“If you really look at [women in film] in an academic way, it can be kind of depressing. For what I do, you have to have an attitude of – you don’t want to think that the whole thing is a mess. I’m trying to keep that sense of trying to do something great and it’s all going to work out.”

Fanning is talking, obliquely, about the sexist portrayal of women on screen, but the closest she will come to explaining what she means is to compare today’s industry unfavourably to that of another era. She looks to the 1940s, “when female characters in film were really crazy, or shockingly aggressive, or there were unmarried women in the workplace. I don’t know whether it’s audiences or filmmakers who want characters to be likable today, but I don’t think actors are afraid of their characters being unlikable. Unlikable female characters, where you see their flaws and bad qualities, but where you see they’re honest and unreal, aren’t unlikable.”

Crepe Showmanship dress by Mary Katrantzou. Photograph: Erik Madigan Heck/The Guardian

Sure enough, Fanning will start filming an adaptation of The Bell Jar next year, in which she will play one of the most complicated and least “likable” fictional heroines of all time, Esther Greenwood. As high-profile roles such as these keep coming, I wonder how good she is at handling her financial affairs – whether she owns or rents her apartment, for example (rents, she says, but knows she needs to buy), and if she’s a businesswoman. Fanning looks momentarily squeamish. “That is a part of life I don’t really like dealing with. It’s never been why I do what I do. Obviously, it’s necessary. I have people I trust who do it for me.”

It is as close as Fanning comes to a shutdown, and makes me wonder how she handles disappointment, mixed reviews or roles that went elsewhere. “Alone,” she says, and laughs. “Alone. I am able to say, this sucks, but I’ll get over it. I will get over it, and move on to the next thing. I’m very able to let things go. The thing I remind myself of is that, whatever you’re experiencing one day, it’s just going to be a memory and that gives me a lot of comfort. You think back five years ago to the thing that was life-defining, and it wasn’t. You moved on. It may still be touchy or painful, but it doesn’t define you.”

She sounds so preternaturally adjusted that I wonder if anything makes her anxious, at all? Fanning thinks for a moment. “It comes out in the weirdest of ways, like packing: I cry when I pack. Nearly every time. The preparation period for anything, be that a personal trip or a professional one – I get so stressed out. I’m someone who can handle more than most people, but then, if you fill it just a little bit too much, I get really overwhelmed and very immobilised like that. I can handle six months of shit, and then the anxiety will come out if my errands pile up, having to pack, having things at the dry cleaner that I haven’t picked up.” She says this with the humour and detachment of someone whose life has been spent identifying precisely this kind of detail as a clue to what lies beneath. “Those little tipping points.”

• American Pastoral is released on 4 November.

• Stylist: Samantha McMillen at The Wall Group. Hair: Bryce Scarlett for Wella Professionals (The Wall Group). Makeup: Quinn Murphy.