BLYTHE Duff does a lot of thinking in her car.

Sometimes, while driving, she will run through her lines, and at the moment that means mentally rehearsing her role as Isabella Stewart in The National Theatre of Scotland's forthcoming James Plays: ambitious dramatisations of the lives of James I, James II and James III of Scotland. Isabella was the last of the Mormaer line of native Scottish kings, and recently Duff found herself thinking as she drove about what a nasty, conniving person her character must have been. "I love playing her," she thought, "but I don't like her as a person. She's horrid. She's vile."

Duff often thinks about past roles, too. And when she's driving into Govan for rehearsals, she observes, like a blast from her own past, the occasional sign directing cast and crew to a television filming location.

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This patch of the city was one of her film beats: a part to which she was frequently summoned for shoots during her 21 years' service as DI Jackie Reid in the STV crime drama, Taggart. Duff always thought that she would know when she was missing television when she saw one of those location signs, for some production she had nothing to do with, and felt the urge to follow it. And that's not happened yet, though all the reminders are there. Recently she bumped into a couple of former Taggart costume girls. Sometimes she passes the STV building. And the process of rehearsing for The James Plays, for which the cast are expected to know their lines in advance, "is the closest to being like television that I've had since I left Taggart".

But she's not yearning to be standing in some desolate spot over a fake corpse, still playing Reid. The actress, who was the longest-serving cast member on the show that defined both crime dramas and Glasgow for a generation, is happy now to be cramming in a sandwich between rehearsals; practising her sword-fighting moves, in a rigorous schedule of physical training that she says is one way of "getting paid to get fit".

This lack of regret is not surprising given what happened to Duff's career after Taggart. In 2011, when the show finished, she was well aware that "you couldn't do 21 years in such a role without people saying, 'There's the wumman fae Taggart', and it was getting near to being, 'There's the granny fae Taggart'." At that point she could have sat around waiting for the phone to ring with something new for the lassie from Taggart, but she decided not to. Instead she got proactive, pushed her stage work and set up a theatre production company, called Datum Point, leaving the small screen behind.

It worked. The year 2013 - in which she turned 50 - was "probably one of the best" in her working life. She received an "extraordinary" review in New York for Beautiful Burnout, in which she plays the mother of a boxer, and "incredible reviews for my one-woman show, Ciara", David Harrower's play about the daughter of a Glasgow crime baron who runs an art gallery, which was written specifically for Duff. Her personal life was also blooming. "My girls [daughters, Katie and Sarah, now aged 20 and 22] started to flourish in a way that I could be really excited about," she says. "And my husband, Tom, took on a new job. So I'm in a nice period in my life."

Isabella is one of the key female roles - the other is acted by Sofie Grabol, of The Killing fame - in a trilogy which is set to be the theatrical event of the referendum year. These plays may ostensibly be about men, about a string of Jameses, but they are written by a woman, Rona Munro, and the female characters more than hold their own. Duff's Isabella is, to all intents and purposes, effectively the queen of Scotland when James I returns to take power.

Duff describes her as "vibrant, vile, vivacious and predatory": a character with "very few redeeming qualities". The plays, she says, are "bawdy and in-your-face". According to Duff, "Isabella spends a lot of her time drunk, just absolutely smashed, which is quite interesting for me because I don't drink". In fact, Duff has never touched alcohol. "Don't

drink. Never been drunk," she says. She admits that "deep within" this is probably about control. It's an element of her character she is open about, just as she admits to being "blunt" and "vocal" (particularly with her two daughters). Being up-front and direct is all part of her down-to-earth charm, though she says she is learning not to be too controlling. Setting up her own production company has taught her "to step back and let people who can do it really well, do what they do". Her husband says she has learned "to be a helicopter", hovering and holding back, before darting in.

The James Plays are, says Duff, "an incredible piece to be doing at this point in history", but they are not propaganda, arguing for Yes or No. "Rona has presented it very cleverly in such a way that nobody could claim it for either side. Of course, people will come along and think they can. But there's no obvious line." The plays will be performed the night before the referendum and also the night after. "Can you imagine," says Duff, "going on stage after we know the result?"

Duff's father - a "stoic disciplinarian" who was in the Merchant Navy for 20 years - was an SNP supporter, and used to carry the young Blythe on his shoulders to political rallies. But although she now credits many of her values, including a strong "working-class moral code", to him, she herself doesn't have "a political head" and has no strong view on independence. She does, however, have an opinion on the "carping and sniping" around the debate, which she hates.

"I know how important being Scottish is to me," she says, adding that she never "flinches" from her Scottishness. That's evident in her acting. Many of her roles strongly convey place, and in particular Glasgow (she was raised in East Kilbride). Ciara was a case in point: among the many ecstatic reviews Duff received was one by the Guardian's Lyn Gardner, who described the play as "specifically about Glasgow, its streets, its tribes and loyalties".

Duff identified strongly with Ciara, a "tough cookie", the daughter of a criminal, who is grieving and craving love. Indeed she says, Ciara has still "not left the building". She feels intimately attached to her in a way she has done with no other role. Partly this is because Harrower wrote the play, an exploration of loss and family that plays out like a Greek tragedy, for Duff, with her voice in mind. "Every now and again I worry, 'Oh I can't remember that bit of Ciara','' she says. ''That's because I need to let her go, but I can't let her go." When Harrower texts her to say that "France is translating Ciara", there's a bit of her that despairs. "Someone else is giving it another life?" she says. "I don't want to imagine her life away from me."

The character of Isabella is Duff's first pre-20th-century role. "It's interesting," she says, "because I think I've got a period look. I've never really thought I look 'now'." When, she was younger, she notes, she didn't look particularly fashionable or glamorous. "I look back on Taggart and think it's a credit to ITV that they didn't want to keep moulding Jackie into some glamorous thing, because that wouldn't have worked."

Jackie Reid was created in a different era - long before it became fashionable for crime dramas to place tough female detectives in a mire of murder and sexual violence; long before The Killing, The Bridge, Happy Valley, Top Of The Lake; long before, as Duff puts it, "the really gritty detective dramas" of today. "Now,'' she notes, ''we get really excited about the fact that women are allowed to be dour and to be really real, and I'm thinking, 'Oh I was doing that for years and years'."

Reid arrived on the scene in Taggart in 1990 (a year before Prime Suspect introduced us to Helen Mirren's DI Tennison), and she grew in professional status as the programme progressed. Partly, it was Duff who pushed that trajectory. "I always fought her corner," she says. "When I think about early Taggart and Jackie Reid knocks and enters and I would basically be saying, 'Sir, something is happening' and walk out the door. Then eventually I got through the door and I got to stand and listen, and then I got to stand and listen and say something at the end of it."

Reid was a career woman - unlucky in love, committed to her job, child-free - and Duff shares some of her character traits, most notably drive and ambition. She recalls that as a younger woman she always felt she could see herself "as being a grandparent and I didn't know whether or not I would be a parent". But she got the kids anyway when she fell for the man next door, Tom Forrest, a real-life detective who had two children. Their mother had died shortly after giving birth to her second child. Hence, Duff became a mum.

"I inherited my daughters," she says, adding that if her family hadn't arrived "ready made" in this way, she wouldn't have been able to remain so focused on her acting career. Not giving birth to her daughters meant that she "could remain physically fit and also emotionally able to the job". She also knew she had the back-up, that Forrest was there making the dinners and picking up the girls from school. Duff and Forrest married in 1998. A few years later he quit his job as a detective to look after his daughters, who were still in primary school. It was, she says, a big deal for him to leave being a detective and decide to be at home. "You have to have quite broad shoulders for that. Because it's not the norm."

Forrest has now retrained as a massage therapist. As a couple they defy traditional gender expectations. People express surprise that she doesn't cook. "People still say to me, 'What does it mean you don't cook?' Women in particular are furious. I don't know whether it's jealousy or whatever they're just like, 'I can't believe you don't do that.' I think it's gradually becoming more normal now. But even 10 years ago it was still considered odd."

That's not to say that there weren't tough times. There were the years when Duff's parents were frail, and she was one of those women who spend their time "running their parents here and there - you can just see them looking at their watches, taking an elderly person, putting them into the doctors, taking them to hospital". She did all that - balanced looking after her girls with looking after her parents. Recently it dawned on her that "I'm past all that now". Her father passed away in 2003, her mother ("zany, good-looking and immaculately dressed"), in 2008.

Recently Duff's agent, Aude Powell, died and it came as a great shock. "It's had an extraordinary effect," she says. "It just keeps creeping up on me." She pauses and collects herself. "I'd been with her for a very long time and she was an extraordinary, powerful person. I was crying for weeks about it."

She's thought a lot about why this loss had such an effect on her, and now says: "An agent is an extraordinary person in your life, you have an incredible relationship with them but you don't know if they take sugar in their tea." Powell made Duff feel part of her family; they spent time together in France. Most importantly, she knew Duff well enough to understand why she didn't want to pursue another television job. "After Taggart," she says, "I needed to be creative, I needed to be allowed to be creative. I needed to find my feet. I needed to push myself."

Duff remains grateful for Taggart. Her years in the series made her a recognised figure at home and abroad, but it also allowed her the opportunity, rare in acting, to have a secure family life, and come home to bed most nights. Unlike many women in the industry, she has, she says, no qualms about her age. "I don't fret over it. I have lived for 50 years and I've had 50 years of laughter, of sadness, of fun, of creativity, of energy, of everything that you can have. I have filled it at every point. I never look back and think, 'I wish I hadn't done that'."

The James Plays are at the Festival Theatre, Edinburgh as part of the Edinburgh International Festival from August 10-20, and at the Olivier Theatre (National Theatre of Great Britain) in London from September 10 to October 29