THERE are few subjects Louis Theroux hasn't tackled. Between his celebrated Weird Weekends series and his BBC Two specials, the award-winning documentary film-maker has covered everything from survivalists to white supremacists; porn stars to criminal gangs; and Neo-Nazis and child psychiatry to paedophilia and Scientology.
That's not forgetting his When Louis Met... format, for which he was granted access to the Hamilton family and Jimmy Savile. It even sent him on a quest to pin down Michael Jackson for an interview.
Now Theroux – whose films have always been produced via the BBC – is back with Mothers On The Edge, a feature about postpartum psychosis.
With statistics showing that in the UK alone, around one in five new mums experience a mental health problem before their baby's first birthday, the 48-year-old will spend time in two specialist psychiatric units which treat mothers experiencing serious mental illness while allowing them to live alongside their babies.
It's a piece Theroux is particularly pleased with.
"I'm someone who started out doing slightly tongue-in-cheek, funny programmes, and then over the 25 years or so I've been working in TV, one of the things I'm most proud of is that I've made this long range, kind of evolution into more serious forms of programme making," he says, barely pausing for breath.
"And in the last few years, that's included programmes about serious mental health issues. So about anorexia, eating disorders, forensic mental health, transgender issues, autism, dementia, and now this one."
Immersing himself on the wards, Theroux meets four women who have been admitted with serious conditions including depression, anxiety and psychosis – often triggered by birth or the strains of motherhood.
As he follows the patients and their families both in hospital and recovering at home, he explores what lies behind their recent crisis and discovers the immense challenge in caring for two people in the most vulnerable state.
Theroux said he felt passionately about the subject, having three sons – aged 13, 11 and four – at home.
"As you get older and you have kids, it's one of the things everyone says, 'Nothing can prepare you for it'," he muses, having married their mother, and his longtime partner, Nancy Strang in 2012.
"And you're like, 'OK, fine, whatever'. Then you do it and you're like, 'God, nothing really can prepare you'.
"All the extremes of emotion, the highs and the lows... And to someone who hasn't been through it, it's hard to explain the sleep deprivation, the disruption and sense of failure that you have so much of the time. Anyway, maybe that's just me?"
The show highlights an extreme form of the familiar, he says.
"Like any mental health issue, it's not that it's taking place in this foreign country or this other planet," he notes. "I found, in the midst of this world, which is to some extent hidden and unspoken about, a great deal to relate to.
"Which I think is maybe the message of the film: that these things should be talked about more and they go into something very deep that's common to many people."
Not every new parent finds it traumatic, however. But, he adds: "If like many people in their 20s or early 30s, you've just been doing your job, watching box sets and going to the pub, and then suddenly you've got someone saying, 'You know what? I'm going to wake you up four times in the night, with demands that can't be satisfied'...
"And then I'm going to flip the script and find some other way of confusing you and make you feel like you're a failure... Well that's normal parenthood. Add to that some experience of actual psychiatric instability, it's a powerful cocktail."
Would he have covered the subject if wasn't a father himself?
"I don't think I would have had the confidence," he confides. "When I was growing up, I used to find babies quite frightening – a bit like time bombs or something. But then you learn a few basic parenting skills, where you go, 'That often works'.
"And talking like this..." he says, giving his best soft baby-friendly voice. "I'm quite a sucker for all that stuff; I actually like it more than talking to grown-up people."
Theroux, who resides in LA with his family, is only too happy to be filming back in the UK, where he was brought up, having been born in Singapore to an American father and English mother.
He credits his "revelatory" 2016 film Drinking To Oblivion with reigniting a desire to work in the UK, too.
"I'd always thought, 'Well, at some level, maybe there's a chemistry between myself and American contributors and it won't work in quite the same way here'," he explains, having previously filmed most of his documentaries in the US.
"But in fact, what I found was that when people are familiar with me when I go into one of these places, it makes for an interesting dynamic.
"I have to be on my game, to be completely honest and upfront," he adds. "As there's a sense in which the relationship is accelerated; they sort of know me and then I get to know them. In some way it's more of a level playing field."
But the subject – or desired subject – of the show he would like to do next could well see him returning across the pond.
"We're not talking in the realms of necessarily the realistic [but] I think a really interesting story would be Melania Trump," he suggests.
"Could she have known what she was signing on for? Did she just think she was marrying a billionaire who she thought was perhaps quite funny? And interesting. Let's allow for that... Maybe not physically attractive, but he could provide her with a certain lifestyle, introduce her to a world that was interesting?" he asks.
"And then now, she's a prisoner of – I'm speaking hypothetically because we don't really know – but she's a metaphorical prisoner of that lifestyle?"
But whether or not that particular interview happens, it looks like his career will continue down the more serious route.
"You know, when you're in your early 20s, as I was, there's more of an inclination to crack jokes and look for funny stories," Theroux says of his early career.
But then, he reflects, "you slightly run out of road with funny subjects".
"The choice I faced at a certain point was to make programmes that are less funny, or perhaps make programmes that are funny but perhaps less interesting. So I went with the more interesting."
:: Louis Theroux: Mothers On The Edge airs on BBC Two on Sunday May 12.
