Oldham’s Sarah Lancashire is one of our best-loved stars. She spent months preparing for her latest harrowing role, which is on our screens this month.

Great British Life: Polly (Sarah Lancashire) with Angela (Joanna Scanlan) and Greta (Eiry Thomas) (c) Simon RidgwayPolly (Sarah Lancashire) with Angela (Joanna Scanlan) and Greta (Eiry Thomas) (c) Simon Ridgway (Image: Copyright © Simon Ridgway, 2019. Channel 4 images must not be altered or manipulated in any way. This picture may be used solel)

Sarah Lancashire's no stranger to on-screen challenges, but despite the sombre subject matter of her new drama The Accident, in which a community is devastated by a disaster, the greatest test was honing the Welsh accent.

'I did a lot of work on it. A lot. It was really challenging. It was awful. In fact, we started filming this in April and just to give you an indication of how long it took me, I had my Christmas dinner speaking in a Welsh accent as I started last November,' admits Sarah, 54, who grew up in Oldham with her brothers.

Her father, Geoffrey, was a scriptwriter, who wrote numerous episodes of Coronation Street, her mother, Hilda, his assistant. After graduating from the Guildhall School of Music and Drama, Sarah combined theatre work with teaching drama at the University of Salford. And then in 1991, she debuted as ditzy Raquel in Coronation Street. Over the next few years, she made an unforgettable impression as the barmaid whose heart was as big as her bouffant, but it's now 23 years since she left the cobbles, bar a one-off appearance in 2000.

Through varied roles in the ensuing years, she's cemented her reputation as one of the nation's finest actors. Along the way, she's been nominated for five Baftas, and won two, for Happy Valley and Last Tango in Halifax, which are both written by Sarah's long-time collaborator, Sally Wainwright.

Great British Life: Sarah Lancashire as Polly and Joanna Scanlan as Angela in The Accident (c) Warren Orchard/The Forge/Channel 4Sarah Lancashire as Polly and Joanna Scanlan as Angela in The Accident (c) Warren Orchard/The Forge/Channel 4 (Image: Warren Orchard. Channel 4 images must not be altered or manipulated in any way. This picture may be used solely for Channel 4 pr)

Last year, Sarah appeared in the acclaimed drama Kiri, as a social worker involved in the case of a murdered girl. She made such an impression on the screenwriter Jack Thorne that he wrote the leading role in The Accident with her specifically in mind.

'I always try and resist writing for specific actors, for fear of heartbreak, but I couldn't keep Sarah out of my thoughts as I wrote this,' said Jack when the four-part series was announced. 'She makes the worst lines sound good, and the slightly better lines sound amazing, and everything has such raw brave truth to it, basically she makes me a better writer.'

Sarah plays Polly Bevan, a softly spoken hairdresser and the wife of local politician Iwan (Mark Lewis Jones) who has advocated a large construction project nearby. It's deemed a source of hope and regeneration, but their daughter Leona, along with a group of her friends, sneak onto the building site, and when an explosion results in fatalities and lifelong injuries, the grieving community seeks justice.

'I didn't actually know the role had specifically was mine, but it was very easy to say yes,' says Sarah who's also appeared in the likes of Clocking Off, The Paradise, and most recently MotherFatherSon alongside Richard Gere.

'I think with most things Jack writes, and particularly in this piece, there's something fearless about deliberately shining a light on that really difficult place where the pubic, political and private collide.'

It's the third drama in a trilogy, following National Treasure, starring Robbie Coltrane, and Kiri, but where National Treasure is about shame and Kiri explores blame, The Accident examines the search for justice, and how difficult it can be.

'I mean this in the nicest possible way, but Jack does this extraordinary thing where he takes a huge story and makes it tiny. So, he does exactly the opposite of what most people do and then he distils it further and examines it forensically through the eyes of those who are most affected,' says Sarah who has three sons of her own, and is step-mum to her husband Peter's three children.

It's incredibly emotive as Polly and the other parents can only hopelessly watch as the building collapses onto their children, but of course, given the way these things are shot, the actors had nothing to go on bar instructions as to what the viewers will ultimately see, from their director Sandra Goldbacher.

'They're always interesting to do because you're not reacting to anything, but Sandra is a brilliant storyteller on and off the screen, and you know when you're getting to those scenes, you're really getting to the meat of the piece.'

The viewers are also privy to private moments behind closed doors, and we witness the true nature of Polly and Iwan's relationship in disturbing scenes. 'Those were challenging but that's the best place to be. Mark's just a phenomenal actor, so I feel very privileged to have had this experience with him,' says Sarah who is currently filming another series of Last Tango in Halifax before it returns next year. After all these years, and despite the continued success, Sarah admits she can never be truly sure how finished project will appear on screen. 'When you're filming, it's impossible to know, but this, The Accident, works.'

The Accident starts on Channel 4 on October 24.