"When you get the call saying, 'Do you want to be in Broadchurch?' you don't really go, 'Well, what's the part? Is there an issue attached to it?'" says Julie Hesmondhalgh. "It was just an amazing phone call to get."

The actress – still recognisable to most as Coronation Street's Hayley Cropper – is sat in a trailer in a car park in West Bay, Dorset. We're on the set of the final series of Broadchurch and Hesmondhalgh is explaining how, when Chris Chibnall comes calling, you just say yes.

The part she was offered and accepted, without an audition, is that of Trish Winterman – a recently divorced Broadchurch local and the victim of a sexual assault that takes place at her best friend Cath's (Sarah Parish) 50th birthday party. Her assailant is unknown and the task of tracking him down falls to trusty duo DI Alec Hardy and DS Ellie Miller, thus setting David Tennant and Olivia Colman's detective pairing one final case before the series wraps up for good as Chibnall moves on to take the reins of Doctor Who.

"It's always daunting when you are trying to represent something that people have gone through and are going through, all the time, every day in real life," says Hesmondhalgh. "The very first scenes that I did were in the immediate aftermath of the attack, and we were in the SARC (Sexual Assault Referral Centre). It was a bit of a baptism of fire but they had advisors – they had a police advisor and a lovely lady from Dorset SARC – so it was reassuring to have them there to make sure I was on the right track. But you always feel a weight of responsibility in that, you know?"

The scenes in question, aired in episode one, show viewers a blow-by-blow account of the process a sexual assault victim undergoes when they report the crime to police. In a country where, according to charity Rape Crisis, only 15% of those who experience a sexual crime go to the police and just 5.7% of reported rape cases result in a conviction, Hesmondhalgh sees the drama as a means of encouraging women – and men – to "feel safe in coming forward".

"I think there is an assumption – that's based on facts, actually, for many years – that you will be treated quite badly when you go to the police and that a couple of male coppers will not really believe you. I think they very much wanted to turn that assumption around and show people how, after years and years of campaigning from people within this field, that it's actually getting better for people and there's a process in place.

"It's patchy, and the level of service is very different in different places, but actually, always you will be treated as though you are believed in the first instance and treated with respect and gentleness, and I think that's something they were very keen to get over. And I think they've been very careful about what they want to show and it not being too sort of 'telly'."

Surrounded by Broadchurch veterans, it is Hesmondhalgh's performance that leaves the lasting impression in episode one. Muted and ashamed as a result of a harrowing attack, she brings to Trish a quivering vulnerability that will tug on you long after the closing credits. 

But in person, Hesmondhalgh is full of energy. From the moment she walks on set, she's chatting away, mouthing off on this and that – from her accent ("I've just had a month off... I came back and couldn't do it any more! I had gone proper back into Farmer Giles") to her hair ("I don't like it. It's divorcee burgundy, we call it"). Most of the cast stayed in a hotel in Bridport but Hesmondhalgh eschewed the offer for a caravan park on Dorset's famous rugged clifftops. "I had a little swim in the sea and I'd walk over the cliffs to work. They'd say, 'We'll send a car,' and I was like, 'No, I'll walk.' It's been really perfect." 

Despite remaining one of Britain's most-loved – and best-known – soap actresses, Hesmondhalgh says she was recognised "very little" on her hilltop campsite. Yet rewind to 2014 and the 46-year-old was hitting headlines and winning awards for the emotional exit of Coronation Street's Hayley Cropper – the first transgender character to be featured in a British soap back in 1998. 

"It ended just as there was this whole movement of young, and not so young, trans actors coming through, and it would have been such an anomaly... it would have been absolutely wrong of me to be playing that part now. It just happened at the right time that I went and everyone else kind of took over and it felt the right moment. It would have been anachronistic for me to be playing that part now, it would have been strange. But I am really proud of all that and it changed my life beyond measure." 

Since Hesmondhalgh's departure she has gone on to land a clutch of coveted roles, appearing in Russell T Davies' Cucumber and series two of the acclaimed Happy Valley, as well as two stage roles at the Royal Exchange in Manchester. But Broadchurch could mark a change in gear in a career that is already going from strength to strength. 

"I never expected I'd be here in a million years. I was happy just to do regional theatre, a little bit of telly here and there – I just wanted variety and scope – but I couldn't have imagined this.

"There's always a list of people that I would eventually like to join when they say, 'Oh, how will it be for them when they leave [Coronation Street]? It's only a handful that have done well...' They always say, 'Suranne [Jones], Sarah [Lancashire] and Katherine [Kelly].' Maybe one day they'll add me on to this list."

Many would argue Hesmondhalgh is already there – but for now she says she has to "work very hard at feeling like I'm not a competition winner.

"The first day [on Broadchurch] I was a little bit overwhelmed by it. We did a photoshoot the other day and it was like being in Madame Tussaud's! I was like, 'What the hell am I doing here?' I had breakfast with Lenny Henry this morning. It's like I'm living in a parallel universe where my daughter is off to secondary school and I'm FaceTiming her and then I'm like, 'So how are things, Lenny?' It's just kind of weird – but I think that would be weird for anybody."

Hesmondhalgh and Henry – who plays local farm shop owner Ed Burnett – are among a brace of star-studded Broadchurch newcomers including Sarah Parish, Charlie Higson (playing Trish's ex-husband Ian) and Bafta winner Georgina Campbell (as ambitious Detective Constable Katie Harford). Old favourites also remain in town, with Arthur Darvill, Jodie Whittaker and Andrew Buchan all returning for the drama's swansong alongside Tennant and Coleman. 

And show bosses will no doubt be hoping Broadchurch can hit upon the winning formula of series one which – at its peak – pulled in 9.3 million viewers and became a cultural touchstone during its eight-week run. Once again, there is a mystery assailant with the identity of the perpetrator kept from the cast until the last possible moment. 

"We've all got our theories and some of them are outlandish," says Hesmondhalgh. "The conversations that we've been having every day I think are going to be exactly the same conversations that will be going on all over the country after every episode. Sarah Parish is pretty sleuth-like and David's good. David made a guess very early on that I think might be right – he was the only one who said it and I think we're all coming round to that now."  

With a new crime and a returning cast slimmed down to our old favourites, series three feels like a fresh start – a return to its traditional form after a thorny second series that dawdled in courtrooms and was met by underwhelmed reviews and disappointed viewers.

"For me, apart from the fact that there are all these regulars in it, it does feel like a standalone," says Hesmondhalgh. "We're all pretty excited about it and feel like it's going to be really good."

Based on a promising first episode, Broadchurch may yet find itself back in the public's affection. 

Broadchurch returns on Monday 27th February at 9pm on ITV