The Scottish-born actress may have been playing a leading role on primetime telly the night before in the first episode of three-part mini-series The Bletchley Circle but, as she sits munching on a salad in the garden of the Union Chapel, Islington, you'd never guess.
Stirling is on a lunch break from rehearsals for Mike Bartlett's new contemporary version of Medea, which opens the Citizens Theatre's new autumn season in a co- production with Headlong. In the early September sunshine, however, with her claret-coloured hair tied up, she could be any north Londoner seeking sanctuary in the Union Chapel's leafy quietude. Only the much-thumbed script in front of her, with the words "Why am I here?" scrawled across the first page in big inky letters, is a give-away.
"It's the most amazing, exciting – I could lick the script – extraordinary part I've ever, and probably will ever, play," Stirling gushes in the throaty, jolly-hockey-sticks voice she picked up at boarding school. "It's not going to get better than this."
Part of the appeal, one suspects, comes from Bartlett updating Euripides's tale of Medea's ultimate revenge on an adulterous Jason to a modern-day suburb. "Medea is a spurned woman, but she's also incredibly clever," Stirling observes. "It's really rare you're playing a Professor Higgins rather than an Eliza. You're the one with the great brain and the greatest articulacy, the one with the greatest ability.
"Yet, for all her brilliant wordsmithery, there's something incredibly simple about Medea, in that she's a woman spurned. There are aspects of her which are naive, which is unusual. She's incredibly powerful, but she's isolated in her community, and even within her circle of friends, which a lot of quite strong or clever women are. But she's funny. You want to be her friend, but you want to give her a wide berth, and there is an element of magic about her.
"She's got this strength of mind, a passion, a fortissimo, a duende. But what I love about it is that all of this is motivated by the absolute simplicity of a broken heart."
Stirling's own hyper-articulacy, laced as it is with street-smart slang and swear-words aplenty, makes her ideal too for Millie, the posh-girl socialite she plays in The Bletchley Circle, in which a quartet of Second World War code-breakers are brought out of hum-drum post-war retirement by team leader Susan, played by Anna Maxwell Martin, to track down a serial killer. Julie Graham and Sophie Rundle complete the circle.
"I love those women," says Stirling. "Julie has been one of my best friends for years; Anna and I, and sweet Soph, we were as dirty as anything. We were very vulgar, like a bunch of hyenas. My Bletchley bitches, babe, I f****** love 'em."
Stirling and co had been on breakfast TV earlier in the week to punt the show, but there hasn't been a glimmer of recognition since. "I was on the Tube this morning, thinking, 'Did anyone watch Bletchley last night?'" she guffaws. "But it's always been a bit like that."
It was the same last year when she worked behind the bar at a friend's pub. She'd just split up with her boyfriend, was being offered a lot of mediocre scripts, but needed to keep busy. If any of the punters asked her if she'd been on the telly, Stirling would deflect them by telling them they must be thinking of Martine McCutcheon. "I was cagey," she admits. "Only in case – no offence – some b****** journalist came in and wrote this thing about 'lesbian star works in pub shock'."
Stirling is talking, of course, about Tipping The Velvet, Andrew Davies's racy small-screen adaptation of Sarah Waters's novel, which combined lesbianism, Victorian music hall and posh frocks with ratings-friendly aplomb. The programme left Stirling and co-star Keeley Hawes exposed in every way. Not that this helped Stirling's career much.
"No one would touch me with a barge-pole," she says. "We didn't take it seriously, but if you're naked and painted gold while wearing a dildo, it's probably safe to say no-one else is taking you seriously either. No-one told me that. I wasn't afraid of the content, but I wasn't prepared for how sensationalised it would be."
All that was a decade ago, however, since when Stirling has been nominated for two Olivier Awards; the first for a supporting role in Michael Wynne's play The Priory, at the Royal Court in 2009, then again a year later playing Lady Chiltern in Oscar Wilde's An Ideal Husband in the West End. On top of all this, Stirling is the daughter of Diana Rigg, who similarly straddled a classical stage career with popular film and TV, most famously as leather-clad secret agent Emma Peel in The Avengers. Not that Stirling has ever exploited her family ties, though she's never denied them either.
"When I got an agent and told her I was Diana Rigg's daughter, her face fell," she says of being picked up after playing Desdemona in Othello with the National Youth Theatre. "I think she thought she'd found some left-field thing, and it's such a boring cliche to be the daughter of an actress and to become one as well. I think she sighed a little."
Stirling and Rigg, their names a detective agency in waiting, filmed a yet-to-be-screened Dr Who adventure in Cardiff last year. Not that Stirling can talk about it.
"I can't tell you anything," she says, "except to say that it's camp as Christmas, and we don't take ourselves seriously. Mark Gattis wrote a script that royally takes the p*** out of both of us, and is totally irreverent.
"I didn't know anything could be that camp, but it's rather moving at the same time. Babe, I saw the Tardis, that's all I'm saying... I'd have to kill you if I told you anything else."
Long before her encounter with the Tardis, a teenage Stirling saw her mother play Medea 20 years ago. "I don't remember it brilliantly, but I do remember the effect it had. I didn't know the story, so, aged 15, the concept of a woman killing her sons hadn't occurred to me. It's not in your canon of awareness. I remember... ooh, that was disappointing," she says, as the plastic fork she's just stabbed into her salad snaps with the force of her assault, distracting her from the tale.
Stirling says she didn't know she wanted to be an actress: "I knew I had to act, but I didn't know what form that would take. I'm also a live-in-the-moment kind of lady. I wasn't the sort of girl who'd sit there at 15 planning what theatre I was going to be working at by the time I was 20. I'm really s*** about saying what I'm going to be doing in a year's time. Or a week's time for that matter."
Even so, by the time she was studying History of Art at Edinburgh University, Stirling was filming ageing-rockers comedy Still Crazy, followed by Iain Banks adaptation Complicity. "I think I got slightly on my tutor's tits," she admits, "but I didn't want to narrow my options, and I needed to get a degree."
Despite such a safety net, Stirling clearly plays by her own rules. While she was working in the pub, she chanced upon The Butterfly School, a charity that teaches literacy to children in deprived areas in London, and ended up running Saturday morning sessions there. She only mentions this when asked what she might do if she hadn't become an actress although it illustrates too how "I quite like to disappear". It's this singular sense of self-possession, one suspects, that makes Stirling both so grounded and such a free spirit.
"I'm familiar with the world in a way I wouldn't be if my mother wasn't an actress," she says. "That's where my knowledge about the profession is helpful, because I don't feel any crazy sense of competition. I know that you can be the most photographed star one minute, and unemployed the next. You can be the world's most famous person, then you go out of fashion. Everybody has peaks and troughs, but your career doesn't define your life. Your life defines your career, I think."
Medea is at the Citizens Theatre, Glasgow, from September 27 to October 13, www.citz.co.uk. The final part of The Bletchley Circle airs on STV on Thursday at 9pm