She has a reputation for being aloof (which she denies), complicated relationships (with both sexes) and an onscreen chemistry with her X-Files co-star (that still has fans guessing). Who is the real Gillian Anderson?

Ask Gillian Anderson a simple question and you get a simple answer. For example, why – after a quarter of a century playing Agent Dana Scully – is she now leaving The X-Files?

“Because.” She leaves the single word hanging in the air. It is a complete sentence, not the beginning of one. She softens the finality of it with a quirky, lopsided smile. This is Anderson to a T – accountable to no one, enigmatic, slightly abrasive, yet captivating and very likeable.

So there you have it, the inside story: “Because.” Filling in the gaps around this word, Anderson turns 50 this year and has long said she wants to do other things, not least spend time with her three children. She has been playing Scully for half her life, even though The X-Files is not the sort of programme she watches. “It’s not my cup of tea,” she tells me. “I tend towards documentaries.”

She is unique to interview, an odd mixture of spikiness and openness. For example, she bristles when I use the word “detached” to describe the characters she sometimes plays: “What do you mean, ‘detached’?” – yet the next moment she is throwing out extraordinarily intimate details, telling me, for example, about her love affair with a woman who died of brain cancer.

Her appeal has more than stood the test of time; if anything, it has increased. In 1996, Anderson was voted “Sexiest Woman in the World” by men’s magazine FHMand, 20 years later, in 2016, she was named one of the World’s Most Beautiful Faces by People magazine. In January this year, her star was unveiled on the Hollywood Walk of Fame. As Scully, Anderson is the glamorous face of rationalism, the perfect foil to Agent Fox Mulder’s open-minded belief in the paranormal. It is hard to emphasise now quite how groundbreaking and iconic the role of Scully was. “You have to remember that what was on TV was Baywatch,” says Anderson. “That sort of [female] character did not exist before.”

We meet at Fox Studios in Los Angeles, which is launching the latest season of The X-Files, the US sci-fi drama series that started in 1993 and ran for 202 episodes over nine years, turning Anderson and co-star David Duchovny (Mulder) into global stars.

In 1997, Anderson became the first woman to win an Emmy, a Golden Globe and a Screen Actors Guild award in the same year. When the original series ended in 2002, Anderson continued her run of powerful, intelligent, complicated (if not detached) women in period dramas (Bleak House, War & Peace); big-budget series (The Falland American Gods); in movies (The Last King of Scotland, Shadow Dancer); on the stage, in A Doll’s House and, most recently, in A Streetcar Named Desire, starring as Blanche DuBois, an uncharacteristically fragile role for Anderson that she performed to devastating effect, garnering her an Olivier nomination.

The night before we meet she appeared on James Corden’s The Late Late Show, telling a very risqué story about one of her sons (she has two, aged 9 and 11) having an erection. She seems, at times, to have no filter.

The woman who appears in the doorway of the Fox conference room looks nothing like flame-haired Scully. With short, almost white-blond hair, Anderson looks like an ethereal version of Marilyn Monroe. She is wearing a sleeveless black trouser suit, which shows a glimpse of cleavage, and totters in on black Jimmy Choo stilettos, clutching – somewhat incongruously – a paper plate with a large wodge of white bread stuffed with rare beef and orange cheese. “This is what I wear to eat roast-beef sandwiches,” she jokes. “I’ll be changing for pudding.”

She picks at the sandwich, eating the beef but discarding the cheese and most of the bread. “Sorry,” she says, as crumbs fly everywhere. “I’ve got to eat. I had oatmeal at 6am [it is now early afternoon] and nothing since. So I’m just getting a little light-headed.”

On sexual relationships with women: ‘It’s just who I am. I have no issue with it whatsoever’

At first we sit on opposite sides of the meeting room, a large coffee table between us. It is awkwardly far away and she pats the cushion next to her on the sofa. “Come over here. Have a seat.” For someone who has a reputation (perhaps unfairly) for frostiness, she is surprisingly warm.

Fourteen years after the original series ended, The X-Filesreturned last year for a short tenth season. Ratings-wise it was a success, although critically it was dismissed as somewhat limp – so limp that Anderson, who had said she was leaving then, returned for a last hurrah. “I think we all felt it wasn’t our finest moment. Chris [Carter, the writer] said it felt like we were learning to walk again. It felt like we could find a better way to end.”

But this really is the end, at least for Anderson – and possibly for the series. Carter has said he will not continue without her. As for Duchovny, he has said he is “good either way”.

While Anderson is about it, she is leaving another hit show, American Gods (she only signed up for one series) in which she stars as the god Media, providing some of the show’s best moments, especially when she morphed into Ziggy Stardust-era David Bowie. “We were very lucky to have her as Media,” Neil Gaiman, the author of the book on which the show is based, tells me. “She is an astounding and subtle actress – one of the finest actresses we have on the planet at this time. But she is in this strange place where everything exists in the shadow of Scully – yet she is bigger and better than that.”

Fortunately for Anderson fans, there is still the possibility that she will reprise her role in The Fall as DSI Stella Gibson, the steely detective pitted against a remorseless killer.

Surely the word detached somewhat applies to Gibson? “She’s certainly cold,” agrees Anderson. “She’s more serious, contained and enigmatic than Scully.”

Of all the characters she has played, she feels that her own character is most closely aligned to that of Gibson. “Although she’s more serious. In my life there’s a lot more levity and, er, silliness, for lack of a better word.”

Scully, on the other hand, is “very passionate and multilayered and goofy and vulnerable”. But “she feels very square to me”, says Anderson.

It is the passion, of course, that most interests viewers – specifically the relationship between Scully and Mulder. In this final season their bond seems stronger than ever – in one scene in episode three they are cuddling in bed [platonically], and Scully asks Mulder if they will spend time together when they retire. “I’ll come push your wheelchair with my wheelchair,” replies Mulder. “That’s not what I mean,” sighs Scully. “What if you meet someone younger who wants to have kids?”

“There’s something quite unique about the connection that David and I have on screen,” says Anderson today. “It’s a weird thing. It’s more evident to me now than it ever was just how important our dynamic is for the series. Fox asked us to come up with our favourite episodes, so I trawled through some and I was struck with the intimacy between them. And I got why people are so obsessed with seeing them get together in some way.”

So if this is her final season, is she allowed now to say if they end up together? “I can’t say yes or no. In fact,” she laughs, “I can’t say that I actually know.”

Off the show, however, Anderson says their relationship is purely professional. “We get along well but in our daily communication, there’s no intimacy. We don’t see each other. Our kids have never met each other.”

Since last year Anderson has been in a relationship with Peter Morgan, the writer of The Crown. Previously she was married to Clyde Klotz, an X-Files assistant art director and the father of her daughter, Piper, now 23. They divorced in 1997, with Anderson saying he bored her. Later she married Julian Ozanne, a documentary film-maker, but they divorced in 2006. She has not married since, although she had a long relationship with the businessman Mark Griffiths, father of her two sons, Oscar and Felix. They parted in 2012.

Of Morgan, she says it is refreshing to be with someone in the same industry. “I’m constantly grateful for the symbiosis, because I haven’t had that experience before.”

I’ve always felt like an outsider. Partly being an only child for so long, and being a Yank in England

It’s been a few years since Anderson first mentioned, almost casually, that she has had relationships with women. “It’s just who I am,” she says today. “I have absolutely no issue with it whatsoever, and I don’t really care if other people have an issue with it.

“We’re all intersectional, and that’s something that is only just beginning to become accepted or embraced in any way, shape or form. We’re all complicated beings. That is my truth and that is my experience and I have absolutely no shame about it.

“The reason I initially came out is because one of the women that I had been with had just died of a brain tumour and I had never spoken about it. I spoke about it in a way to honour the fact that it existed and to honour her existence in my life.” Were they still together when she died? “No. This was way back when I’d just graduated from college … It was a really important thing for me.”

Although Anderson has a tight-knit group of friends, who sometimes tease her about how little time she has to spend with them, she says that she is just as happy in her own company. “I like being a hermit. When I was working on The Fall, on the rare occasions that I had days off, I wouldn’t leave the hotel. I would do everything, including yoga, in my room and just be completely self-contained.”

As a child growing up between London and the States (her parents are American but she lived in London between the ages of two and eleven), she says she felt different. “I’ve always felt like an outsider. I’m sure part of that was being an only child for so long [she has a much younger sister and a brother, who died, aged 30, of a brain tumour]. And part of it was being a Yank in England. Even though I felt British, I was referred to as a Yank. When you’re told that you’re ‘other’ or that you don’t fit in or belong, it’s got to have repercussions.”

At high school in Grand Rapids, Michigan, Anderson was voted “most bizarre girl” and “most likely to be arrested”. She was in fact arrested on graduation night, for breaking into her school to glue the locks of the doors. She went to college in the States and has hopscotched between the two countries ever since; she has perfect English and American accents, which she adjusts seamlessly, depending on the company. Is she living in England now? She scoffs impatiently. “Now! For 15 years.” Well, that includes now. “Yes,” she concedes, smiling, “that does include now.”

When she started out as an actress, Anderson says she was quite “elitist” and had no intention of doing television. “What I wanted to do were BBC dramas and Merchant Ivory films. But what do they say? ‘The way to make God laugh is to tell him your plans.’ ”

In other words, she found herself sucked into television when she auditioned for a sci-fi pilot. Would she have signed if she knew how long the role would last? “Ummm,” she says, and pauses so long that I think she is never going to answer. “It’s tricky,” she continues eventually. “When I signed up, all I knew was that I could pay the rent. There’s part of me that would say, ‘Of course, it’s an extraordinary character and I’m extraordinarily lucky.’ ” But there were times when she felt as though she had signed away the best part of her life. “We shot 24 episodes over nine and a half months every year, over and over and over and over again. Shooting [each episode] of that kind of series in eight days is insanity …”

In 2002, when the original X-Files ended, Anderson appeared first on stage in What the Night is For, then later in the BBC dramatisation of Bleak House, playing a superb Lady Dedlock – “a Ming vase with a Munch scream”, as one critic memorably described her. “Then it felt like I was back on the track of what I was built to do,” she says.

For years, Anderson has been saying she wants to work less. “It’s not really in my nature to slow down, but I’m thinking about taking on very little this year. I’ve got a couple of big things on the books for 2019.” She is not allowed to say what they are. She is involved with a dizzying array of charities and also writes sci-fi books. Most of all, she wants to spend time at home with her sons before they become teenagers. “I want to catch them before that turn happens and they don’t want to have anything to do with me.”

But she finds it hard to switch off. “I don’t relax and enjoy the moment, although I try really hard when I’m with the kids – especially now they’re getting older and one doesn’t have to chase them round the house or outdoors as much. There’s more room for relaxation, because one of them might actually pick up a book and maybe we could read in the same room …”

I express envy that she can persuade them to read a book at all. Is she strict about gadgets and social media? “The first [son] got a phone for Christmas, so this is all quite new. But yes, I am quite strict …”

As for turning 50 soon, Anderson says, “I feel good about it. I’m quite … is excited the word?” She looks unconvinced. I guess there’s not much one can do about it, I comment somewhat obviously. Anderson laughs. “Unfortunately there’s not, try as hard as we might.”

If she were to give any advice to her younger self, it would be to think carefully before saying yes – to anything. “It was always very difficult for me to look at the big picture of things: to step back and judge whether something was right – whether it’s work or a human being. There are a few things in my life – and no, I’m not going to get specific – where I wish I’d had enough self-belief and self-worth to sit back and say, ‘Actually, is this a good idea?’ ”

Her daughter, Piper, she says, is the opposite. “She really considers things; she pauses before she leaps. And certainly my leaping has had benefits. With theatre, I know that if I think about it too much, I will backpedal. If it’s something that I know is going to challenge me in a certain way, I will say yes and go for it – even though I’m absolutely terrified to my boots. So that is an example where it pays off. There are other examples where my leaping is less, er, productive.”

She feels most like her character in The Fall: ‘Although in my life, there’s a lot more silliness’

An outspoken feminist and supporter of the Time’s Up movement, Anderson was furious when she discovered that she was offered half of what Duchovny was being paid for the 10th season of The X-Files. “[I said] ‘You either pay me the same or I’m not doing it’ – which means they don’t have a show.” Most women, she says, are not in such a privileged position. “My [position] does not equate to everyone else’s – where [women] have their livelihoods on the line and the stakes are so much higher. I have huge respect for anybody who has the balls to stand up and say, ‘This is not fair,’ and I think there is now a loud enough conversation that there is a shift taking place.”

Perhaps the most surprising thing about Anderson is the sense of calm she emanates. She is very easy to be around; unflustered and poised. It is no surprise to hear that she practises meditation as well as yoga. “Sometimes I wake up and realise I haven’t done it for two weeks and I’m like, ‘What happened? I lost it’ – it’s as if I left my purse behind. And then I realise that I’m stressed or handling things differently, so stepping back into it is as much for the benefit of everyone around me as it is for myself.”

Before meeting her, I had listened to a 2003 Desert Island Discsrecording in which she sounded like another person, talking about her struggles with depression and drugs and her surprise that she hadn’t died in her teens. “Oh, I should listen to that again,” says Anderson, agreeing that she has changed. “I think children have a big impact on one’s existence. Weathering life carves one’s personality.”

When I ask where she sees herself in 20 years, she says she doesn’t really think ahead, but then offers: “I would like to direct again [she directed an episode of The X-Filesand often bemoans the lack of female directors], but I slightly question my patience. I was recently directed by a friend of mine and I kept asking myself whether I would be able to behave as graciously as her … I suffer from great intolerance.”

And on a personal level, she is even vaguer. “I could be in a relationship with a woman next year.” For the moment she is very happy with Morgan, but makes no assumptions about the future. “I think it’s important that it’s a daily choice,” she says of her relationship. “Part of me looks long-term – as long as there’s always a back door.”