Billie Piper and Lucy Prebble have been close friends for nearly 15 years, and it shows. Speaking to them together, it’s easy to feel like a spare part, smiling politely as they swap stories, laugh at each other’s jokes or turn the interview into a two-hander by posing each other questions. Since they first met, in 2006, when Prebble wrote ITV’s Secret Diary of a Call Girl with Piper in the title role, they have shared a work life as much as a social life, and now it seems that they share almost everything. There’s an honesty and an ease between “Bills” and “Luce” to the point where you sense there’s no question you can ask them that they haven’t already talked over themselves.

Really, though, if you want to know what’s going on inside their heads, go and watch I Hate Suzie — it’s all there on screen. Piper, 37, plays Suzie Pickles, a former child star who is now an actor, performer and mother to a young child. We meet her just as she is offered the role of a lifetime, at which point her life falls apart — minutes later she discovers that her phone has been hacked. A set of compromising photos are already grubbing round the Internet. The series follows her over eight episodes as she disintegrates and then slowly rebuilds. Each of the episodes is branded with a different emotional waymark, from “shock” to “denial” via “anger” and through to “acceptance”.

Since they first met, in 2006, when Prebble wrote ITV’s Secret Diary of a Call Girl with Piper in the title role, they have shared a work life as much as a social life, and now it seems that they share almost everything.

“Bills and I started talking about all those women who had their photos stolen a few years ago,” says Prebble, 39, referring to the 2014 “Celebgate” iCloud hack, “and how we never really heard very much after that about what happened. That’s very modern in the sense that there’s this massive thing, and it’s salacious and horrible and fascinating and technological. But then it disappears very quickly. And we began to think about what those people would have gone through privately. The more we started to think about that, the more we thought, this is a really good way of exploring issues of modern life, femininity, performance and anxiety as well.”

Although they stress that I Hate Suzie isn’t autobiographical, it’s obviously personal territory for them both. Piper was a teenage pop star, morphed into a TV star on Doctor Who and is now a respected actress with three young children. She knows well the acrid cocktail of adulation and censure that is modern celebrity.

“I don’t know what that’s like [having pictures stolen], but I certainly know what it means to live your life publicly. I’ve done that since I was 14. What’s really interesting and slightly depressing is on some level we all do that now — everybody has an online profile and everything you do on some level could be seen to be incriminating, even if it’s actually quite innocent.”

Piper plays Suzie Pickles, a former child star who gets offered the role of a lifetime—at which point her life falls apart.

Prebble says that when they were in the writing room scribbling story ideas on cue cards they had a special piece of paper reserved for things that they thought were too explicit, too rude, just too much for TV. That piece of paper — Prebble thinks she may still have it in a bag — ended up providing most of the ideas that made the cut.

“There’d be a point when Billie and I were in the room and one of us would say something and the other one would go, ‘Yeah, but we can’t do that.’ As soon as we said that I’d say, ‘That’s where we’re going.’”

In an age when you can “do” almost anything on television, from Game of Thrones’ busting of bonk to Fleabag’s searing confessional, you wonder what taboos are left. Having watched a few episodes of I Hate Suzie, however, I can see they have a point. One scene in particular, with Piper on the toilet, contains sound effects the likes of which I have never heard before on television.

Piper’s part in particular is the sort of near-the-knuckle role that gives agents nightmares. Then again, you would have said the same about her starring role in Prebble’s 2012 play The Effect. “If I find myself feeling wary [about a script] that usually means I find it interesting,” she says. “And this show only works in the me-and-Lucy combination. I can’t imagine anyone else writing it.”

One scene in particular, with Piper on the toilet, contains sound effects the likes of which I have never heard before on television.

“You’re brave in your career choices,” Prebble says, again ousting me as interlocutor. “You’re very courageous in what you want to do…”

“Maybe,” Piper says. “I just like doing things that feel honest to me. That’s why I’ve always preferred roles in theater for women because they’re always way more well-rounded and they’re not afraid to show women as mean or mad. They’re not just pleasant sidekicks — I hate that.”

Each episode of I Hate Suzie is branded with a different emotional waymark, from “shock” to “denial” via “anger” and through to “acceptance.”

The danger in all of this is that it becomes a love-in. If, as they say, I Hate Suzie is “the television show we would make if we could actually do what we wanted to do”, if that means the writer and her friend-cum-muse sat in a room together and simply brain-dumped anecdotes on to the screen, wouldn’t that all be a little too insular?

“We’ve also done a lot of work individually,” Prebble says. “We’ve done a lot of personal reflection and therapy and all of that. When you’ve done enough of it, that gives you a nice springboard.”

Neither of their paths to success has been a cakewalk. Prebble says that after Enron in 2009 she struggled. The play, about the financial collapse of a corporation, transferred from the Royal Court to the West End and was nominated for six Olivier awards, winning best director for Rupert Goold. But it lasted only a month on Broadway, and as Prebble entered her thirties there was a number of years when the work dwindled. “I was having trouble writing, finishing anything, I was slightly spiraling downwards with confidence.” After that slump, however, she flourished with The Effect and A Very Expensive Poison (2018), and recently with a writing and producing role on HBO’s hit Succession.

Piper’s thirties have been similarly changeable. She has gone from TV star to acclaimed theater actress (with The Effect and then as the lead in Simon Stone’s reworking of Yerma), has given birth to two young children, divorced from the actor Laurence Fox (she had previously been married to the presenter Chris Evans) and, last year, directed her first film. No wonder that when she and Prebble came together to write they landed on an exploration of thirtysomething womanhood.

“Our experiences have been different for both of us, haven’t they, Luce? But overall they feed into something very similar — coming to terms with who you are, where you came from, how that’s impacted your life, your mental health. It’s, ‘Why do you keep doing this? Where are you headed? Why haven’t you achieved enough? Why am I so restless?’ All questions that for me and people I know seem to start emerging around those early thirties years. For me it was like a journey back in time, coming into my thirties. I was forced to look back, and some of it has been quite unsavory.”

As such, I Hate Suzie is for both of them a series of unmaskings. Eventually Suzie is a woman stripped of any aura or pretense. “The character is an actress, but that’s not to do with the fact that Billie is an actress,” Prebble says. “It’s more to do with the fact that we talked a lot about persona and the playing of roles. In your thirties you can’t really pretend to be someone different from who you are because it’s more pleasing and easier and convenient.”

I Hate Suzie explores issues of modern life, femininity, performance, and anxiety.

Naturally, things get dark, fast. Piper describes their shared sense of humor as “twisted” and adds: “There’s a comfort in mining the darkness between us as well. It’s not like, ‘If I suggest this to Lucy is she going to be shocked?’”

The only matter on which they differ in opinion is technology. Prebble’s plays, from The Sugar Syndrome onward, are interested in how tech affects truth and perception. She is “obsessed” with technology and has an active Twitter presence. Although I Hate Suzie shows the dark side of social media, she thinks it gives people such as Piper a voice that wasn’t available 20 years ago when she was tabloid catnip. “There was a time when the press, and press barons, millionaires you’d never meet, would make the decisions about what your narrative was going to be.”

For Piper, though, social media is merely another source of anxiety. “Let’s put it this way, I wouldn’t have wanted to be a famous pop star now. The social media element of it all is terrifying because it’s continuous, it’s in your bed, next to your bedside table every night. In my pop career there were moments that were horrible to experience as a teenager, but they came and went. But the thought of having that constantly on your person — I wear my phone round my neck like some weird slave — and with one simple click you’re able to see what millions of people think about you or other people, or just about anything… I would argue that it’s got worse.”