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“I might have to breastfeed,” Sienna Miller says matter-of-factly, providing proof, should you need it, that she’s one cool mother. She disappears from her Zoom window where, moments before and much to the delight of myself and Dominic West, her co-star in Sky Atlantic’s glossy new legal drama War, she was holding her baby son. “He’s so alert and interested,” West says. “He loves the screen,” Miller replies. “Which is worrying.”

On screens, Miller has been replaced by a photograph of a rodent. “That’s a nice guinea pig,” I say, thrown because I’m a man and no one has ever breastfed during an interview I’ve conducted before. Off-camera and sounding a little distracted, Miller replies: “That’s a hamster.” When she reappears, her baby is horizontal, disrupting the stripes of his mum’s Chanel top. The hamster, it transpires, “didn’t live long”.

I wonder how Miller and West feel about getting older. “My life doesn’t feel particularly old at the moment,” Miller, 44, says, “but I enjoy getting older. Given how much tragedy there is, it’s a privilege. And there’s a relief in not being as attached to people’s opinions. I don’t see it in terms of ageing. It’s life lived, challenges overcome and new chapters.” Miller sniffs her baby. “And little boys! Oh, I’m in love!”

West, who turns 57 next week, is less enthused. “I’m nearly f**king 60, which is properly old,” he says, before conceding that acting may be uniquely suited to arresting that process. “In a way, you have to preserve a sort of childish innocence, because you’re just playing. The older you get, the more you realise it’s an absurd job, but I’m grateful to be able to do an absurd job and get paid for it. I keep meeting people who have got proper, grown-up jobs and are responsible, and I’m still doing what I was doing when I was eight.”

What the pair are doing now is publicising their roles as film star Carla Duval and tech billionaire Morgan Henderson, whose divorce proceedings spark a bitter battle between two rival London law firms. Judging by War, created by George Kay, whose previous credits include Lupin and Hijack, divorce is a manoeuvre undertaken to avoid being suffocated by one person only then to be devoured by several people instead.

How did they prepare for their parts? Did Miller have any reservations about playing a selfish, needy, manipulative actor? “I was very newly pregnant when I got this script and very, very pregnant when we finished filming it, so the whole thing is a bit of a blur,” she says. “I wasn’t too fazed by the idea of playing an actress. I probably should have thought about it more, but it was quite clear from the start that we were very different people.”

For Miller, what appealed was this difference. “Carla’s quite cold and quite calculating, and I like the idea of toning down my own vulnerability, turning off the people-pleasing side. It was very liberating to not care what people think of you as a character.”

To play a billionaire, West credits the BBC Sounds podcast Good Bad Billionaire with sharpening his view of the mega-wealthy as a class. In it, BBC business editor Simon Jack and journalist Zing Tsjeng survey a billionaire’s life and work, and then decide if they’re good, bad or just another billionaire. “Of the world’s roughly 3,500 billionaires, three are benign,” West notes, referencing Yvon Chouinard, founder of the clothing company Patagonia, and the late Chuck Feeney, co-founder of Duty Free.

When I suggest a third, it’s met with a resounding “no” from West, followed by a “but that’s off the record” from Miller, who clearly doesn’t want us to be sued by someone that wealthy. “I think inherent to being a billionaire you’ve got to have a ruthlessness,” West says. “Most of them are among the worst people on the planet. Who’s that dude? The tech guy. He’s awful.”

To research the darker mechanics of a high-net-worth divorce, West “talked to a guy who was a rich man who’d got divorced, and I asked him how brutal it gets. He said, ‘It gets absolutely awful.’ I suppose the richer they are, the more appalling the divorce is.”

What hooked him about Kay’s scripts, other than them being “absolute page-turners”, was the central question they kept returning to: how do people who love each other become people who want to kill each other? “The answer, in this case, is through lawyers.” Miller continues: “War is an honest portrayal of the ugliness and the underbelly of that profession. Having a peek behind the curtain of the process of building a case – the backstabbing, the bribery, the blackmail – you see that it’s Shakespearean, the lengths people will go to. It’s ultimately about money, which is ugly.”

She adds: “I don’t think there’s anyone lovely in the show in the same way that there’s no one lovely in Succession. It’s the world of it.”

As for drawing on personal experience of relationship tumult, neither West nor Miller are strangers to romantic ups and downs. Miller’s early career was overshadowed by her relationship with actor Jude Law. Today, she has two children with her partner, actor/producer Oli Green, and a daughter (Marlowe, 14, who accompanied her to the premiere of War) with her ex, actor Tom Sturridge. West is a father of five children, four with his wife, landscape gardener and aristocrat Catherine FitzGerald.

Miller says, judiciously: “Any life experience, positive or negative, that anyone goes through, we all use as kind of fodder for our work. It’s amazing to have an outlet like this. Actually, the more pain and the more joy and the more extremity of emotion you’ve experienced in your life, probably the better resources you have to dig from. I think you become a sum of the experiences of your life, and that enriches you as an actor. I don’t think in this case that I personally was drawing on one particular experience.”

Pointedly, smiling, she concludes: “Having never been married or divorced, it was all just fun and games for me.”

For West, War highlights the competitive nature of marriage. “The people we play are corporations. She’s a big film star and he’s a big businessman. They compete on almost a corporate level, but you can see that in any marriage or relationship. There’s a loving concern for each other, but there’s also a flip side, in that it can be highly competitive, can’t it? I’m just trying to choose chairs with my wife, and it’s this real competition to be right and be more right than her. Or is it just me?”

Something West and Miller didn’t have to work on for War was their chemistry, which is evident from the moment West appears slightly late on-screen for our Zoom meeting. “There he is, swanning in on his own clock,” Miller snipes lovingly. The pair first worked together more than 20 years ago in a West End production of As You Like It alongside the late, great Helen McCrory.

“It was my first ever play, so I was 21 when I met him,” Miller remembers. “It was an incredibly bonding experience and there is love there. It’s not that we’ve seen a lot of each other over the years, but when we have, I’ve felt such fondness – which is actually perfect for constructing a marriage.” That history, she says, did much of the work when it came to their chemistry.

With a candour that characterises the whole conversation, West agrees. “It’s dead easy to be in love with Sienna; you don’t really have to act any of that. We had this ease with each other, which you get after doing a play for months in the West End. The harder task was manufacturing the opposite.” The conversation turns, inevitably, to the kind of characters West has built a career on playing – men, as he puts it, who let him “be pretty awful”. Where does Morgan Henderson sit between Fred West and Noah Solloway, his roles in Appropriate Adult and The Affair respectively? “Somewhere in between – a ruthless bastard who’s trying to do good.”

Playing real people, West says, carries a different weight to playing invented ones – something he felt keenly as Prince Charles in The Crown and even more so as Fred West in Appropriate Adult. “You usually feel a duty to argue for your character to be a better person than the writer wants them to be. With Charles, you don’t have the freedom that gives you – everyone already has a fixed idea of him.”

Even with West, the serial killer he played to widespread acclaim, he admits to the same reflex. “Unfortunately, I think it’s true even with murderers. You’re trying to find something human in them, something you like.” He says he no longer wants that kind of darkness. “I definitely feel I don’t want to play people like Fred West any more. I’m much more interested in good people now. Maybe that’s the state of the world, maybe it’s getting older.”

For Miller, the pivot point in how she was perceived professionally came a decade ago, when she appeared in Bennett Miller’s Foxcatcher and Clint Eastwood’s American Sniper, both Oscar-nominated in 2015. “It definitely put me in a place where I was allowed to be seen as someone serious,” she says. “I’d had moments of that before, but they’d been overshadowed by a lot of toxic early-2000s fame.”

She is frank about how strange the early 2000s, when she was tabloid fodder due to her relationships and her fashion sense, now look in retrospect. She is also surprisingly sanguine about how appallingly she was treated by the tabloids; she was awarded substantial damages from News of the World in 2011 and News Group Newspapers, publisher of The Sun, in 2021. “I was watching that Mourinho documentary [on Netflix] and seeing the paparazzi in it, remembering how that was just how people lived. It’s so dated. It’s like smoking on a bus.”

On the subject of time passing, I ask how becoming parents has changed the choices they make in their careers. Both actors describe becoming more selective and more protective of their time. “You want to get paid, you want to get home early and, if you live in London, you want to shoot in London if you can,” Miller says. Decisions that once felt “floaty” now have to be weighed against time away from her children – to the point that she recently turned down an extremely well-written play because of where it would take her emotionally. “It was about a woman whose child had died. I just don’t want to sit in that space right now.”

With our time drawing to a close, I ask West about the BBC drama The Hour, in which he starred alongside Ben Whishaw and Romola Garai. It was where he first encountered Kay, who, as a newish writer, cut his teeth on Abi Morgan’s show. Would he return if there was a new series? Without hesitating, he says yes. “I saw Abi quite recently and she said she had an idea to bring it back and set it in 1968, against the backdrop of the year’s events, protests and riots. I think it’s a brilliant idea.”

Miller adds: “As you get older, you just have to be like, ‘I do this because of the experience of doing it. I cannot be too attached to the outcome.’ You can get stuck in the hamster wheel of chasing more success or rewards. I see it happening all the time. You need to remind yourself and check yourself regularly. So I’d like a job, too. Give us a job, Abi!”

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