TV: Jane Campion is back with a second series of the acclaimed Top Of The Lake

Jane Campion - giggly, warm and quietly glamorous - is describing how she and Ari Kleiman, co-director of Top Of The Lake: China Girl, used subterfuge to research the setting for the sequel to 2013's acclaimed Top Of The Lake: Sydney's sex industry.

To get into a brothel, they pretended he was a virgin and she his aunt, who wanted to help.

"We got ourselves into the sex room," says Campion, 63, who won an Oscar for her screenplay of The Piano. "It was understood that he wasn't there for sex, just to experience being close to a woman and talk to her."

Eventually, they found a group of sex workers who were happy to talk about "anything".

"We had the experience of hanging out at this one brothel and they were so generous and funny. It does feel like [they could say] anything, [like] how popular the ladyboys are with the married men."

Top Of The Lake: China Girl starts with the body of a woman (unidentified, so nicknamed China Girl for her ethnicity) washing up on the shores of Sydney in a suitcase, her dark hair streaming from inside.

Mad Men's Elisabeth Moss returns as Detective Robin Griffin, who investigates the case and gets an unlikely new partner in Game Of Thrones' Gwendoline Christie.

Those who watched - and loved - season 1 (it was Emmy and Bafta nominated and won Moss a Golden Globe) will remember we left Robin at the edge of the New Zealand lake, with Johno, who she likes, but who may or may not be her brother, having found missing pregnant girl Tui.

At the beginning of the second series, she's back home in Sydney and clearly unhappy.

"You could have gone anywhere [with it]," says Moss, 34. "You could have had four years later and her and Johno had this great relationship and had a baby, or you could go the way we went, which was not the happiest. I requested that we go in a darker, deeper, more challenging direction, as I really didn't think there was much to do otherwise."

One of the biggest changes this series is the move from the open skies of the New Zealand countryside (where The Piano's Holly Hunter headed up an all-female commune beside the titular lake) to the hustle and bustle of Sydney.

"I just couldn't come up with any ideas set in New Zealand, it felt like the excitement of that we'd exhausted, or for me anyway because that's where I live some of the time and I really wanted to share it," explains Campion.

"But I also live in Syndey and [it's] a really exciting location for me because even though it's not a wilderness, it is a wilderness. The ocean really is that wild quality - the edge of the city that can be like a wild beach, it can be really peaceful, it's really moody, feminine as far as I'm concerned because it's tidal..."

Besides the main investigation, the other big plotline is that of Robin reconnecting with the daughter she gave up as a baby. Now a teenager, played by Campion's real-life daughter Alice Englert, Mary lives at home in a posh suburb of Sydney with her dad, who her academic mum (Nicole Kidman) has left for another woman.

Moss explains: "The second season, far more than the first, is very much about female relationships and the complications and uniqueness of [them].

"It's [also] about motherhood and different kinds of parenting. Robin is technically a mother, but she has never experienced being with her daughter, so she has to find her own way of being a mother to Mary."

Mary finds solace in the arms of 'Puss' (David Dencik), a shady, German academic who lives in a flat with his cats and hangs out with the sex workers. So the question is what does Puss know of China Girl's murder and is Mary in trouble?

Well, the most either the mum and daughter duo will reveal is that some of Englert's scenes were so distressing, Campion asked co-director Kleiman to direct them.

"She found it upsetting to see me upset, which is so cute isn't it?" says Englert, 23. "She's a good mum! I really trust her and it's a pleasure to work for her."

"I just knew I wouldn't be able to manage it," admits Campion. "As a mother, I'm a little bit of an emo. If she's feeling bad, I'm like 'Oh!' I just knew I wouldn't be able to go the distance, [and say] 'do another take'. I'd just be like, 'That's fine, let's move on!' The mother aspect kind of threw me there."

Campion wrote the role of Mary with her daughter in mind, which, she says, was "kind of a brave thing to do".

"I didn't realise how brave it was until I finished it all. I thought 'Oh my God, what if she'd been really bad?' I think she did really really well, but it did suddenly occur to me it could have been not a gift, but a bomb!"

Also joining the cast was Gwendoline Christie, known for playing Brienne of Tarth in Game Of Thrones and Captain Phasma in Star Wars: The Force Awakens, which she'll reprise in The Last Jedi this December.

A huge fan of the first series, she wrote to Campion, saying how much she'd love to work with her.

"She's a friend of a friend of mine and I didn't know her or her work, so I Youtubed Game Of Thrones and thought, 'Oh God, she's amazing!'" says Campion. "And I really thought her and Lizzie, who's 5-foot-something little, with this 6-foot-something partner, would be great."

On screen, they make an odd-looking crime-fighting team, but both Moss and Christie (who plays Miranda) loved exploring the female friendship.

Christie describes it as: "Conflicted, complex, disturbing, unsettling and tumultuous", adding that she and Moss "developed a glorious friendship that allowed us to embrace the work and do everything we could to make it really flourish and mine the relationship".

Moss adds: "Neither of us want to spoil anything for anyone because it is actually a giant arc - it's the main relationship of the season besides the one of me and my daughter so it's very important to me. I will say that they are opposites in a way - the exterior of Miranda is really quite sensitive, open, enthusiastic and passionate, whereas Robin projects this exterior that she wants to be considered strong, fierce, tough, but really she is very soft and vulnerable on the inside. They do connect over what they have in common."

The actress also has a couple of scenes with Nicole Kidman, when Mary's adoptive mother and biological mother meet.

Moss reveals: "There's sort of a battle of the mothers, but my memory of [working with] Nicole is very much that I got a break when she came. She came round for two weeks at the end [of the shoot] and I was so tired and got to have a few weeks off while she was working. I was very happy to see her!"

• Top Of The Lake: China Girl starts on BBC Two on Thursday, July 27