Phoebe Waller-Bridge is sitting in an upmarket restaurant, asking me to teach her how to burp at will. “I can’t armpit fart and I can’t fake burp, and I think that’s a tragedy,” she says. We met about five minutes ago; I’m not quite sure how we reached this point. Waller-Bridge is tall, gangly, preppy. Head-girl material. But a rebel head girl who likes to get everyone into trouble – herself included.

She is trying her best to burp, but nothing is coming. “How d’you do it?” she pleads. I let one rip. “You really commit with your face,” she says approvingly, before trying again. Eventually a little burp peeps out. “That was really delayed,” she complains. “I’ve got good comedy timing with my farts, though.”

It turns out she is a tongue gymnast, too – she can make it do forward rolls, the splits, you name it. And then there is her Hammer House of Horror party trick, where she makes her eyeballs shake in her head. It is truly gruesome. “I remember being very young at school and asking somebody, ‘What happens when I do this?’ And the girl was like, ‘Aaaagh!’ ” Classic Waller-Bridge; she loves to shock.

Take the opening sequence in Fleabag, her 2016 TV series about a young woman trying to cope with loss, make sense of life and find a lover who can satisfy her. The eponymous Fleabag delivers a monologue to camera while a 2am booty call is acted out in front of us. “After some pretty standard bouncing you realise he’s edging towards your arsehole. But you’re drunk, and he made the effort to come all the way here…”

By now we’re in the bedroom with them, and she’s still talking to camera. “He’s thrilled. Agh. Ow! Then the next morning you wake to find him fully dressed, sat on the side of the bed, gazing at you. He says, ‘Last night was incrediblebecause I’ve never actually managed to… up the bum with anybody else before.’ Then he touches your hair, and thanks you with a genuine earnest. It’s sort of moving. Then he leaves. And you spend the rest of the day wondering: ‘Do I have a massive arsehole?’”

Like the rest of the series, it’s funny, shocking, and beautifully written.

Waller-Bridge depicts characters who say the unsayable, do the undoable, and defy every stereotype of feminine behaviour

Fleabag – her family’s nickname for her is Flea – was a success both here and in the US. Now Waller-Bridge has written a new TV series, Killing Eve, based on Observer dance critic Luke Jennings’ thriller Codename Villanelle. Killing Eve, the story of a female MI5 security officer hunting down a young female psychopath (and vice versa), has already been a hit in America, where it was nominated for two Emmys, and is about to be released in the UK.

Although a very different genre from Fleabag, you can instantly hear Waller-Bridge in the snappy, sardonic dialogue. Again, she has focused on strong women – bored, passionate middle-aged Eve and young, amoral sadist Villanelle, superbly played by Sandra Oh and Jodie Comer respectively. The show is crammed with sadistic killings and dismembered bodies. But there is a quiet, startling scene right at the start that sets the tone. A young woman is sitting in a cafe, staring at a little girl eating ice-cream. She smiles at the girl, but the girl doesn’t engage. So the woman puts her heart into it – her smile becomes bigger, more sustained, more heartfelt. And finally the girl responds with a sweet, trusting smile. The woman then leaves the cafe; as she passes, she tips the ice-cream into the little girl’s lap. Not a word is exchanged. It is a brilliant, economic piece of writing.

Waller-Bridge depicts characters who say the unsayable, do the undoable and defy every stereotype of feminine behaviour. Even more astonishing than her ability to depict these women is her knack of making us love them. “I write from the point of view of what I’d like to watch. I’m always satisfying my own appetite. So I guess that means transgressive women, friendships, pain. I love pain.” She grins.

With Hugh Skinner in Fleabag. Photograph: Hal Shinnie/BBC

She scans the menu. “I’m going to have the burrata.” I look nonplussed. “It’s like a creamy mozzarella, everything you want mozzarella to be – pudding for starters.” Is that all she’s having? “Oh no, I’m starting from the top and working my way down.” She orders ravioli, and says she’s going for it today; there will be no holding back.

More than anything, she says, as a writer she wants to show women indulging their appetites and venting their grievances. “We sexualise women all the time in drama and TV. They are objectified. But an exploration of one woman’s creative desire is really exciting. She can be a nice person, but the darker corners of her mind are unusual and fucked up, because everyone’s are.” Has she always been able to say the unsayable? “Yes. As long as it feels truthful, as long as it’s pointing at the elephant, it is always exciting.”

Waller-Bridge grew up in an upper-middle-class family with baronets on both sides. Her father co-founded Tradepoint, the first fully electronic stock market (“He has a huge scientific and creative brain”) and her mother works for the Ironmongers’ Company in the City. (They are now divorced.) Even as a little girl, she says, she loved to shock. “My grandmother was very proper and Christian and buttoned up. I adored her. She was very shockable, and as a kid shockable people were my ideal playmates. She would always want to have a prayer before we ate and I’d always be like, ‘Please can I do it, please can I do it, I’ve practised one.’” I ask for an example. “At school, when I was about seven, I’d heard someone say, ‘Good meat, good God, let’s eat’ and I thought, well, that’s quite fun.” She pauses, apologetically. “It wasn’t outrageous, just slightly irreverent.”

At the age of 11, she was sent to boarding school, and hated it. She lasted a year. When she returned home she sounded so ra-ra posh that none of her fellow pupils at St Augustine’s Priory in Ealing could understand her. (She describes St Augustine’s as if it were one up from borstal, though actually it is an independent school.) “The first weekend they asked if I wanted to go to see a fight. Mum said, ‘What are you doing at the weekend?’ and I was like, ‘Mum, I’m going to a fight in Ealing Broadway’ and she was like, ‘God, how exciting! Marvellous!” Her voice has a lovely singsong quality to it. Waller-Bridge swears like a trooper but punctuates her profanities with words such as “marvellous”. If her life story is ever turned into a movie, Emma Thompson would be a natural to play her.

Photograph: Dylan Coulter. Styling: Deborah Ferguson. Hair: Dennis Gots at the Wall Group. Makeup: Fiona Stiles at starworksgroup.com. Above and top: Helmut Lang dress; H Stern jewellery; Amy Noel tunic; AOTC trousers.

Waller-Bridge was in her element at secondary school. She starred in plays, made people laugh, took ever more joy in tackling taboos. After A-levels she went to Rada to study drama. Rada is famous for breaking people down before rebuilding a new, improved version. But with Waller-Bridge, she says, they just left her in pieces. (They told her she was emotionally blocked because she couldn’t cry on demand.) “I went to Rada thinking I was quite a good actor and came out thinking I was appalling.” She struggled to find work through most of her 20s. That’s one of the reasons she started to write – to provide decent parts for herself.

It was only when she met her great friend Vicky Jones 10 years ago that life started to change. Jones and Waller-Bridge instantly connected creatively. “Everything began to make sense in my life,” she says. They encouraged each other to write, mentored each other, drove each other on, eventually co-founding the DryWrite theatre company. After Waller-Bridge had written a number of short plays, including a 10-minute vignette of Fleabag, it was Jones who made sure they were performed. “We were pissed at the Soho Theatre one night, the artistic director was there, and Vicky just ran up to him and said [she slurs her words], ‘Phoebe’s just written a fuck-load of short plays and I think you should fucking put them on upstairs.’”

'Good things can come out of rage. Once you know what makes someone angry, you can tell a lot about that person'

Fleabag was turned into a one-woman show, directed by Jones, and won the Fringe First award at Edinburgh in 2013. Since then, Waller-Bridge has not stopped working. In 2015 she appeared as junior barrister Abby Thompson in the second series of Broadchurch. The same year, she wrote and starred in Crashing, a comedy drama following the lives of six twentysomethings living as property guardians in a disused hospital. (“It was my homage to shows I’d grown up on, like Friends and This Life.”) In 2016, the six-part series of Fleabag was screened on BBC Three, and in 2017 Waller-Bridge won a Bafta for best female performance in a comedy. As well as writing Killing Eve (in which she does not appear), this year she has played a droid in Solo: A Star Wars Story; last month, it was announced that she was working on a second series of Fleabag, with the original cast, joined by Andrew Scott.

Fleabag tackled taboos we didn’t even know existed. In one scene, she masturbates to a video of Barack Obama giving a speech, as her boyfriend lies asleep next to her in bed. Waller-Bridge believes this is the scene that has most resonated with women. “I was walking through Soho a few weeks ago and a woman was walking past me, and she then kind of recognised me and went, ‘Girl girl girl! You’re Fleabag! You wank to Obama! I hear you, I hear you!’ That’s the scene most women talk to me about.” Has she ever used Obama for inspiration? “Have I ever wanked off to Obama?” She laughs. “I’m going to keep that to myself.” She pauses. “I mean he was just the most exciting man on the planet, and he was devastatingly handsome. Wanking about Obama is for me a perfect joke because it felt real. And there was also something joyous about the politics of the time. He was hope, and I was wanking to hope.”

With creative collaborator Vicky Jones. Photograph: Getty Images

At the heart of Fleabag is a profound sense of self-loathing; during the series, we gradually discover why she is in such a bad way – her mother has recently died; she is alienated from her father whose new partner despises her; and her best friend has killed herself. Where does this theme of loss come from? Waller-Bridge mentions a friend who died when she was 20. “She was pretty close. She was my best friend’s other best friend, so we were always hanging out together. And it was a real shock.” How did she die? “She… killed herself. She was the smartest, most beautiful, funny, cool girl. We knew she was depressed, but she was such a sunshine personality. She was like, ‘This is just a thing I have to deal with…’”

But, Waller-Bridge says, her obsession with loss comes more from a sense of foreboding – the fear of loss. “The idea of losing my mum crept into my mind very early. A friend of mine had lost her mother when I was very young and I just couldn’t fathom it. Still the idea of my mum dying is unbearable. I could cry just thinking about it now.” In the end, she says, she learned how to cry for parts by imagining her mother’s death.

Waller-Bridge adores her family. Her older sister Isobel, a composer, wrote the music for Fleabag; her brother Jasper works in the music industry. “I had a lovely, safe, loved upbringing. Dinner every night, Mum and Dad encouraging us to talk and make each other laugh. I come from a place of security, and that has probably given me the confidence to look at my insecurities in a safer way.” At the same time the very politeness of her background made her want to blow a great big raspberry at decency and convention.

Sandra Oh in Killing Eve. Photograph: Sophie Mutevelian

Waller-Bridge has often said she was inspired by Louis CK, another comedian with the ability to explore the darkest recesses of the mind. “He seemed to see the world through a female perspective, and so much of his comedy was about how gruesome men are and how many women have to deal with gross guys. No one was talking that way at the time – you know the idea that a man goes on a date and thinks he might not get laid; a woman goes on a date and thinks she might be murdered.”

Did she think that? “Oh yes, because that’s the messaging you get. All the training you have about how to be a woman is about defending yourself from very early on. Guys get told to go out, be brave, fight it. For girls it’s like, be careful, always have a friend with you, cross the road if someone’s following you. So you go into the world defensive, because if you don’t do those things you’re likely to be raped or murdered.”

Was Waller-Bridge disappointed in Louis CK? 'Yes, of course. But it won’t take away from how much his work inspired me'

Last year a number of women accused CK of sexual misconduct, and he expressed remorse for abusing his power in the industry, before returning to the stage last month. Was Waller-Bridge disappointed in him when the women came forward? “Yes, of course. I was very disappointed. But it won’t take away from how much his work inspired me.”

I tell her that I’ve noticed some common themes in all three TV series she has written. “Really?” she says. “Do tell.” Well, I say, a little uncomfortably, you seem to have a thing about small breasts. In Fleabag, a lover gets off on telling her she has such “small tits they’re hardly there”. In Killing Eve, one of the first things we learn about the psychopath is that she is small-chested. I bet the references to her breasts aren’t in the books, I say. Waller-Bridge bursts out laughing. “No, they aren’t. Yes, I am obsessed.” Why? “Because I have small tits? I guess I’ve always been fascinated by tits because in some way they are an expression of femininity and they are so important when you’re growing up. Whether you have tits or not, when they’re going to come, the girl who’s got massive ones who doesn’t want them and is trying to cover them up, and the girl who hasn’t got them and is stuffing them with tissue. The stakes are so huge around tits, and I think that remains through life.”

Another obsession is needy boyfriends – handsome, perfectly nice, kind men who are just slightly too weak to satisfy their women. “It’s refreshing to see them, isn’t it? You see it so many times with women.” She acts out an anxious, cloying partner. “‘When are you coming home? Where have you gone?’ Put it in a man’s mouth and it becomes interesting.”

She believes that men suffer as much from gender stereotyping as women. “It’s the pressure of uber-masculinity, and feeling that you have to be the satisfier, the annihilator, the sexy man who can do it at the drop of a hat.” Has she been with a man who calls himself the Annihilator? “No! But I did have one who called himself the Appreciator!” A boyfriend? “No, just a short encounter; I suppose an Appreciator is better than an Annihilator; at least you’re likely to come out alive!”

Nina Ricci trenchcoat; Narciso Rodriguez trousers; Giuseppe Zanotti boots; H Stern jewellery. Photograph: Dylan Coulter/The Guardian

Waller-Bridge showed an early cut of Fleabag to her family, and it was her brother’s response that most interested her. “He said, ‘I think you’re going to scare a lot of men with this show. It’s going to freak them out.’ And I was like, ‘Fucking good, it’s about bloody time.’ Again, that narrative of keeping your man satisfied is rammed down our throats forever.

“Rage,” she says suddenly – that is her subject. “Good things can come out of rage. It’s galvanising. A director once said to me, ‘You have the gift of rage’ and it really stayed with me.” She repeats the expression with wonder. “‘The gift of rage.’ I always wanted to hit the edge of the character. Once you know what makes someone angry, you can tell a lot about that person.”

I rage at myself for my ability to let things slide because I’d rather be ‘nice’ than stand up for myself

She talks about the first time she remembers raging. She was around 14 and was due to meet a friend who hadn’t turned up, so she went around to her house. “I rang the doorbell and her older brother leaned out of the window and told me to fuck off, and said, ‘She’s not coming out.’ He was just controlling her. I wanted to throw bricks at his window and go in and get her.” But she didn’t. She bottled up the feeling and used it as source material years later.

Later, I text her to ask what makes her rage now. “I feel rage about casual and systemic sexism,” she emails back. “I feel rage at how quickly the double standards could be balanced if men gave women the back up they need to stop us having to shout into our own vaginas all the time. I feel rage about how many times I’ve Googled before and afters of chemical peels when I could have been working, when I know I’ve just been programmed to do that.

“But mainly I rage at myself for my own ability to let things slide because I’d rather be ‘nice’ than stand up for myself in an uncomfortable situation. My characters have streaks of fearlessness. I get a rush writing women who don’t care what you think. Probably to help me grow into being one.”

***

Waller-Bridge is a fascinating mix of the restrained and volatile, the shocking and shockable, the tough and soppy. As we order dessert (“I have a rule that if there is a chocolate mousse, you have to have it”), I tell her I’ve noticed another recurrent theme in her work – women who betray best friends by sleeping with their man. She looks at me open-mouthed, horrified. “Is it?” Then she shouts, appalled. “Oh yes!” Has she ever slept with a friend’s boyfriend? “No! That is like going into my fears. It’s because I’m such a romantic. Infidelity breaks my heart. That’s why I write about it, because I’m obsessed with it.”

Her own personal life has gone through great changes. Last year, Waller-Bridge and her husband, documentary film-maker Conor Woodman, announced they were divorcing. “Aha!” she says when I mention it, as if she’s been waiting all this time for me to bring it up. “I don’t really want to talk about my marriage.” Early this year, it was revealed that she was dating Martin McDonagh, director and writer of Oscar-winning film Three Billboards Outside Ebbing, Missouri. If ever there were a couple that seem to be creative soulmates, I say, it’s you two – both of you humanise extreme characters in your writing; test our ability to empathise with monsters. She nods enthusiastically. “The line between adorability and monstrosity,” she says. “I take that as a compliment.” She talks about seeing McDonagh’s play The Pillowman long before they got together. “It was the first play I saw that made me think theatre can be really exciting. Sometimes you need to see something like that – a piece of work that wakes you up, that makes you go, ‘Yes, of course that can be done’ and gives your imagination permission to go 17 million other places. I felt like that about his work.”

With her boyfriend, writer and director Martin McDonagh. Photograph: Getty Images

Is this the happiest she has been in a relationship? “I’m not going to talk about that, either.” She looks at the glass of wine in her hand. “Three more of these, though!” she says. At the moment, she and Vicky Jones are renting a house, having fun together. “She’s about to move in with her boyfriend, and we thought, let’s just have six months where it’s just you and me, baby.” When she is done with Fleabag 2, she plans to take a proper break for the first time in years.

Her immediate priority is to go travelling. That’s the strange thing about success, she says – it’s wonderful because it enables you to do what you always wanted to, but it leaves little time for anything else. “Since Fleabag, I’ve been in a bedroom under a duvet, writing.” When the work takes over, she says, you lose contact with the world. “I really felt that when I was writing Killing Eve. I don’t have material from life to put into this. You need it. That’s why people like your work in the first place.”

Has she been surprised by her success? She nods, and mentions Jones again. “Vicky has now got a pilot with HBO which she has written and I’m exec-producing.” When they first met in 2012, neither of them was writing. “The two of us went out for dinner the other night for the first time in ages. In a way we’ve never stopped and taken a breath. We each got a glass of wine and then she just leaned back in her chair – and I felt this so deep because it’s like six years she’s talking about – and she went, ‘Well, that went rather well, didn’t it?’”

We leave the restaurant. Outside Waller-Bridge gives me a hug and says she’s got something for me. She stands perfectly still, concentrates, and breaks wind. She grins – every inch the rebellious head girl. “Did you hear that?” she says. “Didn’t I tell you I could time my farts to perfection?”

• Killing Eve starts on 15 September on BBC One and will be available on box set on BBC Three via iPlayer.

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