WHEN the first series of Happy Valley aired, it wasn't only the general public who were raving about the psychological drama, but police officers too.
The reaction was "eye-opening" admits Sarah Lancashire, who reprises the role of police sergeant Catherine Cawood in the hit series.
"I had an inspector write to me who'd been in the force for years, and he said, 'That's the most accurate portrayal of a police officer'. Of course, this isn't down to me. This is down to Lisa Farrand, who's our police advisor, and who Catherine is really based on."
Recalling her very first meeting with the former WPC, Lancashire says: "The first thing she said to me was, 'You're going to have to man up'. She's very no-nonsense and just gets on with the job."
The 51-year-old earned a Bafta nomination for the first series (her co-star James Norton was also nominated, as was the director Euros Lyn, while the series won the award for Best Drama Series and Sally Wainwright took home the Best Writer gong), but Lancashire admits the shoot was a "brutal" experience – as the second series has proven too.
"You do enjoy it but they're hard. They're endurance tests, really," she remarks.
"I think the whole thing is draining, simply because of Catherine's disposition. She wears her emotional state of mind everywhere. There's always the weight of it. It's great to play."
She won't be drawn into discussing whether it's difficult to switch off once the cameras have ceased rolling for the day.
"I don't talk about how or why because I loathe doing it. It's really only of interest to other actors," she notes, in her trademark softly-spoken, thoughtful manner.
"Also, it always sounds w*nky on a page."
In series one, viewers watched the psychopathic Tommy Lee Royce taunt Catherine following his release from prison. She, in turn, became increasingly obsessed with the man she believes fathered her grandson Ryan after raping her daughter, who later took her own life.
When the series returns, 18 months have passed.
"Tommy's in prison, [sent] down for three category A murders [for a bungled kidnapping plot in series one], so he's not coming out," explains Lancashire.
"Catherine's well, mentally strong, ticking along quite nicely really, and then she finds a body and the rest of the series unravels from that point."
He might be behind bars but Tommy continues to play psychological games with Catherine, and forms a bond with a mysterious female admirer, played by Shirley Henderson.
Meanwhile, it's believed a serial killer might be behind a spate of killings and two new people are brought in, John Wadsworth (Kevin Doyle) and Jodie Shackleton (Katherine Kelly), to investigate.
"It's very interesting because they are CID," notes Lancashire.
"Catherine loathes Jodie Shackleton, simply because she used to be CID and she knows how CID works."
The scene they're shooting today, on a particularly miserable, grey day in Hebden Bridge, West Yorkshire, is a pivotal moment.
"Catherine and Tommy meet face-to-face once, and this is it," reveals Oldham-born Lancashire.
"I said to Sally [Wainwright, with whom she also works on Last Tango In Halifax], the series could – but it shouldn't – exist without Tommy, because he's such an intrinsic factor.
"I can't see the show without him, otherwise it's a cop show and I don't think it ever should be a cop show. There are lots of those, and what sets this piece apart is that it should always be a good psychological piece of drama. They've ramped that up hellishly this series."
Given the huge success of series one, it's no surprise the BBC was keen to see it return.
"Sally and I had a long chat about it, because we could've just left it. She sort of had to convince me, really, that there was more to tell, and I put my absolute faith in her. When I read the scripts, I thought, 'She's pulled it off'.
"You just hope that we can equal the first series, that's all, equal it."
She has "no idea" why Happy Valley struck such a chord with viewers.
"I'd like to think it was the piece as a whole, that it was so engaging and really captured people's imaginations," says the mother-of-three.
"Clearly people have an appetite for this kind of thing."
The violence was a talking point, including the shocking scene in which Catherine is beaten up by Tommy, which led some critics to question whether the programme-makers had taken things too far.
Lancashire doesn't think so.
"Well, you have to remember she's a police officer, so it's absolutely justified. I would never condone anything that I thought was salacious or titillating or gratuitous, and the violence from last year was brutal. However, I don't think any of the furore would've happened if that had been two blokes," she says.
Asked if Happy Valley is the project she's proudest of, Lancashire, who appears in the new big-screen outing of Dad's Army as Mrs Pike, confesses: "I'm never proud of anything, to be honest."
She won't even watch something she's in.
"I'm too critical. I'm just looking at the choices that I made, or the performance level, and I learnt really early on that it's destructive, it's not helpful for me. I don't want to dissect what I do. It's fleeting, and then you've got to go away and prove it all again. It can be torturous.
"I'm not a neurotic mess!" she adds with a laugh. "I sound like I'm a neurotic mess, but what I'm very good at is compartmentalising. That's work, and this is home."
Lancashire trained at the Guildhall School of Music and Drama, before appearing in a number of theatre productions, and then landing her big break as Raquel in Coronation Street, a role that made her a household name in the nineties.
"There is prejudice against actors who've come from soaps, but it didn't worry me, because I was very aware of it," she says.
"It didn't concern me in the slightest, because television is only one strand of an actor's existence."
After Happy Valley, she'll be taking a long break, and plans to enjoy doing "absolutely nothing".
"I'm very much a mum when I go home [she has two grown-up sons from her first marriage to Gary Hargreaves and a 12-year-old son with her producer husband Peter Salmon]. I'm away from home such a lot doing this."
She's hoping filming should start on the next series of Last Tango in late summer.
"It's been nice to have had a break. The difficulty is getting us all together at the same time, but we're always in touch and texting each other," says Lancashire, who talks of the "special bond" she shares with co-stars Sir Derek Jacobi, Anne Reid and Nicola Walker.
She considers herself "the luckiest person ever" to be involved with "two of the most beautifully created series".
"It's phenomenal," she says. "It's a purple patch because it will go. She [Sally] will do other things and so will I, but it will be a beautiful four years."
It's because of her involvement in Happy Valley and Last Tango that she finds it difficult to wade into the debate over lack of roles for women in their 40s and 50s.
"I realise I'm one of the fortunate ones. I've managed to get through that tricky period with two great roles, but the age issue happens to everybody. Actresses don't have the monopoly on it." It's the same with pay disparity, she adds.
"It's the entire bloody world," says Lancashire. "Not just our profession."
:: Happy Valley returns to BBC One tomorrow night.
