IT is lunchtime in a busy Glasgow restaurant, where Morven Christie and Vicky McClure are chatting like old friends catching up, their loud peals of laughter carrying across the room.

The duo are shooting BBC psychological thriller, The Replacement, just around the corner at the former 2014 Commonwealth Games headquarters in the Merchant City, which has temporarily been turned into the offices of a fictional architectural firm.

The close, off-set friendship forged by Nottingham-born McClure, 33, and Glasgow native Christie, 35, is in stark contrast to the bristling dynamic that viewers will witness between their on-screen alter egos in The Replacement.

"We are having the best time and getting on like a house on fire which is brilliant," says McClure, who made her name in Shane Meadows's award-winning film and TV series This Is England as well as Broadchurch and Line Of Duty. "If we didn't it would be so painful because of the roles we are playing."

The three-part drama, which begins on BBC One this Tuesday, tackles unflinchingly subject matter that includes the fetishisation of motherhood, gender politics in the workplace and the stigma surrounding mental health.

Christie – who has starred in Grantchester, Doctor Who and The A Word – plays Ellen, an ambitious architect whose sense of identity is sent into freefall when she unexpectedly becomes pregnant at the same time as landing the biggest project of her career. Swapping her bricks-and-mortar baby (a £12.2 million library) for one of real-life flesh and blood proves an emotionally difficult transition.

Not least when Ellen's plans to return seamlessly to work after the birth are thrown into turmoil by the arrival of her maternity cover, McClure's character Paula. On paper, they are polar opposites: Ellen has dedicated herself to climbing to the top of her chosen profession, while Paula took a decade-long career break to devote herself to full-time motherhood.

Paula quickly wins over colleagues and clients with her professionalism, charm and boundless enthusiasm, but there is something intangible that leaves Ellen feeling increasingly rattled. The flames of discontent are further stoked by Paula's insistence that when Ellen falls in love with her new baby she won't want to return to work.

As the Hitchcock-style suspense and razor-edge tension builds, Ellen becomes convinced that there are far darker forces at play. Yet, Ellen's family, friends and even her husband dismiss her growing concerns as yo-yoing pregnancy hormones and a jealousy-driven personality clash.

"Quite a lot of the time it is difficult to tell who is being paranoid, whether things are really happening or whether they are imagined, and whose side you are meant to be on," explains Christie. "Like Vicky says, if we were doing this and didn't get on it would be difficult because it is not direct conflict, it's beneath the surface and that can snake under your skin."

At a Bafta Scotland screening of The Replacement in Glasgow last month, writer and director Joe Ahearne revealed that the gripping storyline was loosely inspired by the experiences faced by producer Nicole Cauverien, his close friend and long-time collaborator, when recruiting maternity cover.

I'm keen to get Christie and McClure's take on the potent issues that Ahearne – who previously worked on Doctor Who, This Life and co-wrote Danny Boyle's 2013 feature Trance – has woven into the helter-skelter twisting plot.

McClure sets down her knife and fork, giving full attention to the topic at hand. "Me and Morven don't have children," she says. "We are both playing a role where we have had kids, or are having kids, and we don't have anything to tap into on a reality level.

"However, we are both women, handling new-born babies and it is hard not to feel cooey and maybe a bit broody. It is impossible not to and is just something that feels natural for us. But it doesn't for everyone and that's all right.

"We are both in our 30s and haven't had kids yet and some people might say: 'Oh, it's a bit late'. Everyone is entitled to their opinion. I think we live in a different generation now where if you are healthy and fit enough to have a child at an age you feel is appropriate for you, then go do it."

A fortnight before we meet, in summer 2016 when the Conservative party leadership contest is in full flow, candidate Andrea Leadsom suggested that being a mother makes her a better choice for Prime Minister than fellow contender Theresa May, who doesn't have children. Motherhood, she said, gave her "a very real stake" in Britain's future.

The subject raises Christie's hackles. An edge creeps into her voice and there's a steely glint in her eye. "This whole 'as a mother' nonsense," she says, her nostrils flaring with outrage. "As a human being. We are all human beings. If you can only find your humanity through motherhood then there is something a wee bit wrong.

"To suggest we are lacking something, that there is a part of us that's dead in our compassion or sensitivity until we procreate I think is madness. I'm a compassionate person, Vicky is a compassionate person, I'm sure you are a compassionate person. Having a child or not having a child does not affect that.

"Those qualities in women are there. I have maternal qualities. Whether I have my own children at the moment or not, it doesn't restrict the ability to use those. I use them in my life all the time."

Christie takes a furious bite from the oatcake she's holding as if imagining chomping off the head of anyone who would utter such gibberish.

Beside her, McClure takes up the thread. "Isn't it amazing we are chatting about this?" she muses. "I find it incredible that people are so small-minded. I just don't get it."

We spend several minutes discussing the reality of everyday sexism and misogyny. "There is an obsession with women's bodies, their wombs and waistlines – it is ridiculous," continues Christie.

"In this story [The Replacement] there is a lot of talk about that in the context of what happens when you are pregnant and suddenly people feel like they have agency over your body in some way. They can police what you eat and whether or not they can touch you.

"With or without the motherhood and pregnancy situation, I think that does just happen to women all the time. There is this constant conversation we are having about who's got a boob job, who has had Botox and who hasn't – it is relentless."

It still feels fairly rare to see two women cast in such envelope-pushing roles in a TV drama. "Usually you get a decent role but you are doing it opposite the guy who is kind of more the lead," says Christie. "This isn't like that. It is proper toe-to-toe."

There is the sense of them valiantly fighting the urge to roll their eyes about how women, in general, are depicted in film and television.

"We talk a lot about 'strong female characters'," says Christie. "And out of that comes this idea they have to be a bit kick-ass and that you're not a strong woman if you aren't fighting …"

" … aliens," finishes McClure.

"Or being Jessica Jones [the Marvel heroine]," continues Christie. "This is an architect. She has trained for seven years. That is a strong woman."

It is a theme that Christie will later elaborate on during a Q&A that follows the Bafta Scotland screening. "I think we should be telling difficult women's stories and seeing more flawed women on television," she says. "That is my MO [modus operandi]."

She also reveals that an early announcement about the programme, which alluded to the "darker side of motherhood and working women", almost put her off entirely.

"I thought: 'Away to f***, I'm not doing that,'" she recalls. "It made me not want to do the piece, but then I read the script."

Even when talking about the show, says Christie, there are people keen to pigeonhole her or McClure in a way she feels is missing the bigger picture about the storyline.

"For me it is about identity," she adds. "The clefts you reach in your life where your identity or the way you define yourself is changing and how destabilising that can be."

McClure is no less sanguine when we catch up later on the phone. "It wasn't ever something where I thought: 'I want to tackle women's issues' or 'I'm doing this part because it is about strong females'," she asserts.

"I enjoy playing strong females and [in productions] where women are at the forefront, but for me it was the story and everything that surrounded the script and the characters Ellen and Paula. It has this weird thriller twist to it. I haven't really done stuff like that before."

Since shooting The Replacement, Christie and McClure have been far from the proverbial resting actors. Christie recently wrapped filming on the third series of Grantchester and is preparing to start work on series two of The A Word next month.

McClure, meanwhile, went straight into production for the much anticipated fourth series of Line Of Duty, due out this spring, an all-action role that has seen growing calls for her to be the next James Bond.

"Oh mate, could you imagine?" laughs McClure. Christie fervently shakes her head. "Bond is a t***. You deserve better than Bond," she says.

McClure nods in agreement. "I'm more of a Bourne girl," she concludes. Christie claps her hands in delight. "You would be a great Bourne. Be a Bourne."

"If I had to be either/or I would be all for Bourne," says McClure.

She recalls the high-octane sequence at the end of the last series of Line Of Duty. "Running around with a massive machine gun and being attached to moving lorries was hard work. I'm not the fittest person in the world. I don't go to the gym on a weekly basis. I was glad people appreciated it. That was a nice compliment."

That image is in sharp contrast to the power suits, talon-like false nails and perfectly coiffed hair of Paula in The Replacement. "Please God, don't think I dress like this on a daily basis," pleads McClure as she splays her fingers on the table. "The nails! At first I couldn't do up my buttons."

Christie guffaws with laughter. "I kept asking her to pass me things because it was so funny watching her go like that … " she says, making a pincer movement with her hand like a flailing crab.

McClure and Christie hope that viewers of The Replacement will find themselves flitting back and forth, torn between rooting for Team Ellen or Team Paula, as the drama unfolds.

"It's only once we get to the end of the story we can see why Paula has done what Paula's done and why Ellen has done what Ellen's done – there is a justification for both," says McClure.

"There are certain things that they each do that aren't right or fair or are cruel, but there is a pain that surrounds them so I don't think the audience will know which way to turn until they get to the end. We hope there will be a certain amount of compassion for each character."

The pair are gregarious company when in full-on bantering mode. "Ooooh, I've got two things," says Christie, gleefully as she tucks into her lunch. "I'm the greedy one. She's got her nice little salad …"

"But usually it's not," interjects McClure. "We are both pigs," agrees Christie. They laugh when I remind them that McClure's Line Of Duty co-star, Martin Compston, once joked that she couldn't be more than 20 minutes away from a 24-hour Greggs bakery. "There is a Greggs on every corner in Glasgow, that's why I feel so at home," she deadpans.

McClure's eyes widen when I mention that there are often security guards outside some of the late night opening branches across the city.

"Really? To protect the sausage rolls?" Beside her Christie chips in. "Yeah, from you," she quips.

The actors strike me as similar personalities: no-nonsense, straight-shooters who don't sugar-coat things. The women exchange fond smiles. "Yeah," confirms Christie. "I would say that is pretty on the money."

The Replacement begins on BBC One, Tuesday, 9pm

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