Sitting opposite the quietly dynamic actor was a young woman who, without warning, asked her what it was like kissing Benedict Cumberbatch. The woman was referring to the famous scene in the first episode of the third series of Sherlock, Steven Moffat and Mark Gatiss's 21st-century reboot of Arthur Conan Doyle's seminal detective stories.
In the programme, Brealey plays mousily put-upon pathologist Molly Hooper, whose massive crush on Sherlock, played by Cumberbatch as a dashingly dysfunctional sociopath, has slowly captured viewers' imaginations. With Sherlock apparently returning from the dead in this series, one of a myriad of possible explanations for his resurrection saw Cumberbatch crash heroically through the windows of Molly's St Bart's Hospital lab and fall into her arms for an almighty snog destined to become one of the programme's defining moments.
Brealey's response to her interrogator was blase, although she admitted she "looked shifty" on her Twitter account, which at the time had 64,000 followers. Once the final episode had aired, that figure had almost doubled to more than 125,000.
Brealey is sitting in the Citz foyer on a break from rehearsals of Zinnie Harris's 1920s-set version of August Strindberg's play. Dressed in a green shirt worn over a long-sleeved grey vest and jeans, and with her long brown hair tied back (although it doesn't stop her fidgeting with it), Brealey ponders the response to both the kiss and a scene in the final episode when Molly thumped Sherlock after he was found in a drug den.
"My Twitter basically broke," Brealey laughs, "and I kept on having to reboot it, because I think in the end there was something like 7000 people tweeting me asking about the slap that I delivered to Benedict's lovely face at the beginning of the episode."
After the similar reaction to the kiss, Brealey should maybe have expected it.
"Obviously everyone adores Benedict," she says, "and it was such a James Bond-y moment, but I was standing on a crash-mat, so was slightly unstable. In one take I actually fell off the crash-mat. I slowly slid off like Del-Boy at the bar.
"But you don't often get to do fantasy shots like that. In the script it was just a James Bond-style clinch, but I wanted it to be a proper snog and not a peck. In one take Benedict did the hair ruffle to get the glass out, and in the next take he didn't do it, and I was like, 'Put the ruffle back in. It's really hot!' It looks good, though, doesn't it?"
Brealey isn't showing off when she says this. Rather, her tone is one of utter fan-girl glee, albeit a fan-girl who got to do what most of the show's female fans would like to do. Several times at that.
"Within a minute-and-a-half of that kiss I had something like 6000 new followers on Twitter," she says, "and I had thousands of tweets that night. One of the reasons Molly works is that women who love or fancy Benedict can quite easily imagine themselves as her. So, of course, when he snogged her, they all just …" She pauses. "… lost it. It was such a collective …" - she squeaks to illustrate - "because no-one was expecting it."
While Brealey seems bemused by the attention, there's a sense of responsibility too for a fan-base she defends fiercely.
"I don't think of fans as geeks," she says, "and I don't think of being a geek as a negative, because I'm a geek. I've got quite a lot of young women following me on Twitter, and it's great to be able to have a dialogue with them. I'm a feminist, so it's great to be able to prattle on about feminist things. Bizarrely, I suddenly find myself a role model for some of these girls. At first it was hilarious, thinking 'how can I be a role model?', which sounded like a terrifying prospect.
"They occasionally ask something to do with sexual politics, and I can say, you know, you don't have to shave your hair off if you don't want to. You don't have to do anything you don't want to. Just be your own woman.
"I don't want to come over all po-faced, because ultimately Sherlock is just entertainment, but if I can, I want to try to set a good example."
Brealey, 34, hunches into herself as she talks, keeping her arms wrapped in tight. Navigating her way through an idea, she scrunches her face up, her voice dropping to an almost inaudible level, only to whirl around and offset everything with a dirty giggle that's almost a yelp.
This mix of a shy but steely intelligence punctured by extrovert flashes may have something to do with Brealey's training. She studied both at the Lee Strasberg Institute, the spiritual home of method acting in New York, as well as with master French clown, Philippe Gaulier. This followed a history degree at Cambridge, where she studiously avoided the university drama set.
"I played football and drank," she says. "I went for one audition and panicked in the queue, and literally crawled out of the church they were auditioning in. I ducked down and walked out of the room like a strange frog. It was quite cliquey, all that, and I didn't have the confidence to feel like I could be part of it."
Brealey became an arts journalist, interviewing the likes of Liv Tyler and the Pet Shop Boys for Total Film, The Face and Wonderland, where she was deputy editor.
"I think quite a lot people get into careers right next to the one they really want to do," she says. "I'd always wanted to act, but I'd been too afraid to stick my neck out and risk failing. Then I decided I didn't want to end up being 40 and wishing I'd done it."
Brealey's first professional job was playing a gobby 14-year-old girl in Judy Upton's play, Sliding With Suzanne, directed by Max Stafford-Clark. A couple of years in Casualty as trainee nurse Roxanne Bird followed, as did the role of Judy Smallweed in the BBC's 2005 adaptation of Charles Dickens's Bleak House. She then toured the US and Russia in Dennis Kelly's play After The End, played Sonya in Peter Hall's production of Uncle Vanya, and appeared at the Traverse in Edinburgh in Simon Stephens's post 7/7 play, Pornography. She was the mayor's sex-mad daughter in The Government Inspector, and in 2012 performed naked as Helen of Troy in The Trojan Women.
Brealey isn't sure where the initial impetus to act came from, although, she says, "I think I must have got some love and attention as a small person by doing some acting, and it lodged itself in my subconscious.
"When I was a little girl, about eight, I auditioned for the school play. They were doing Snow White, and I auditioned for the Wicked Witch, and then the teacher asked if I'd like to play Snow White. I responded by asking who was playing Prince Charming."
Such precociousness has clearly held Brealey in good stead for Sherlock, although recent suggestions by a tabloid newspaper of the programme having "left-wing bias" passed Brealey by. Once informed about the reports, however, she makes her views plain.
"Good," she says. "I'm a socialist, so I'm quite happy if it does, especially when you've got rampant Tory propaganda like [Channel 4's] Benefits Street going on. But honestly," she laughs, "I don't know where they could have spotted that. It wasn't something I spotted, but that would be marvellous. I would be proud to be involved in a left-wing drama."
Miss Julie may not quite be that, but Strindberg's sexual cat and mouse game between an aristocratic young woman and the male servant she grew up with is certainly getting there.
"It's one of the great parts written for a woman," Brealey says. "Zinnie's focused on the sexual politics of the play, and we're looking at what happens when boundaries are transgressed, and the things that hold society together are broken. It's about these two people who have this thing between them, and negotiating what that is.
"If you are friends with someone and have sex with them, can it ever be the same again?
"It's set on midsummer as well, which is difficult to imagine in the middle of January, so there's that whole sense of opening a shirt, and your hair's all damp at the nape of your neck, so I'll be wearing a lot of thermals to give my little body the impression of warmth."
Beyond such hidden layers, Brealey is still finding her way into Julie.
"She's unconventional is the short answer," Brealey says. "I really haven't worked out who she is yet, and I don't like to put too much down beforehand, because otherwise you end up closing down possibilities.
"I'm a feminist," she says again, "and it's interesting reading Strindberg's preface to the play, where he talks about Julie as a half-woman, a man-hater and a degenerate, as a type that can't survive in the real world, because they will always come up against failure when it comes to trying to be equal. But you have to remember just how shocking the play was for its time. These people in the play are talking about sex. They have sex."
As an actor, Brealey doesn't take anything for granted. Despite the Sherlock factor, she's aware of the fickleness of her profession. After the first series of Sherlock, she ended up as a researcher for TV documentaries, and created The Charles Dickens Show, a mock chat-show in which Dickens's characters appeared on the daytime sofa.
"It suited my magpie brain," she says, "but acting is my first love, and it's a jealous lover at that. You can't just leave it. I've been lucky, but you have to be incredibly careful about thinking that acting makes you happy, because acting doesn't make you happy. Nothing makes you happy, actually. You've got to try to make yourself happy. Now I've learned that it makes it easier. When the phone doesn't ring, if you let that make you feel unwanted, then you are on a hiding to nowhere.
"I'm happy just to work with nice people. Of course, you want to work with people who make your mind light up, and who push and challenge you and make your work better. That's a given. But I don't want to work with brilliant w*****s. Life's too short."
Last year Brealey wrote a play, Pope Joan, for the National Youth Theatre, about the legend of the ninth-century female pontiff.
"I'm so glad I did it," she says, "but it cost me a lot. It was very exposing, and there wasn't enough time to write it, but I learned a very valuable lesson."
Taking her clothes off as Helen of Troy was even more exposing.
"It was amazing to do that," she says, "because I don't run round in the nuddy as a rule. You're only six or seven feet from the nearest person, and you're going, 'Look at me - I'm beautiful' and you've got your bum out."
Playing Sonya in Uncle Vanya was another learning curve.
"I would play her again in a heartbeat," she says. "I think that part really made me a stage actress. I learned so much from that job, but in a way, in terms of unrequited love, Molly is sort a mini TV Sonya."
Ah, Molly again. "Molly's opened up all sorts of doors for me, but I've been offered about 10 secretaries who are in love with their boss, and I've turned stuff down. After Molly I need to play someone who's a complete gun-toting megalomaniac."
"It's funny, isn't it," she says, "about ambition, because it's a dirty word, especially for women, but I just want to learn. The thing about acting, and it's hard to say without sounding like a tit, but with the best jobs, you learn to be a better person as well as a better actor." n
Miss Julie is at the Citizens Theatre, Glasgow, February 6-15. Visit citz.co.uk.