The first thing you notice about Shirley Henderson is that no one else notices her. Dressed in a demure, plain-coloured shift and clutching a handbag too small to carry more than a lipstick and purse, she slips quietly through the half-empty Glasgow cafe in which we've arranged to meet. She slides stealthily into the seat next to mine. An attempt at a kiss – air or cheek – would, I sense, be an error; Henderson prefers the professional formality of an army officer's handshake.
It would be a mistake to interpret Henderson's understated appearance as the attention-avoiding tactic of a mildly famous celebrity, though she has certainly taken roles that have granted her a modicum of fame – many will know her as the downtrodden teenage ghost Moaning Myrtle in the Harry Potter movies, or the obsessive-compulsive Charlotte in ITV's Dirty Filthy Love, for which she earned a Royal Television Society award nomination in 2005.
Her polite but very precise instructions to the waiter make clear that the 44-year-old is neither shy nor coquettish; she is no-nonsense and businesslike. After ordering she turns to me, smiles warmly and folds her hands neatly on her lap, ready to begin. Something about the way she sets her chin and holds my gaze without hesitation or discomfort suggests a mettle to this stick-thin skelf of a woman.
That might have been what attracted director Todd Solondz when he decided to cast Henderson in his latest film, Life During Wartime, the dysfunctional sequel to his feted 1998 dark family comedy Happiness. Producer Christine Walker certainly suggested that the fact that Henderson had worked with Mike Leigh in 1999's Topsy-Turvy might have prepared her for filming with the equally precise Solondz.
Henderson laughs knowingly. "Yes, precise and exacting, both of them," she says. "They have a kind of wickedness, these guys, seeing how far they can push you, what they can get out of you. Todd's very forward, he likes to get right in your face. You have to be brave." She tells me about Solondz directing a restaurant scene by shouting instructions while he sat cross-legged on the floor under her table. "It's a big pressure. You can't refuse to work that way – you have to step into their world. So it's scary yes, but wonderful when you get it right."
As sequels go, Life During Wartime has the loosest of connections to its predecessor, with characters having shifted in age, background and sometimes even race. The themes of Happiness – paedophilia, sexual betrayal and family breakdown – are all present in the new movie, and even turned up a notch. It's the film Woody Allen might make if he contracted that terminal disease he's been fretting over for decades and saw his artistic vision darken to jet black.
"It's dark, yes," says Henderson. "But the humour was the reason I wanted to do the film. I like doom and gloom with a sense of humour. Maybe it's a Scottish thing, we like to undercut indulgence with a laugh."
Putting Henderson in the vulnerable, childlike role of Joy is a masterstroke on Solondz's part. He exploits every aspect of her – her small stature, milk-thin, bony limbs, line-free doll's face and famously bubble-light treble voice – to make her the quivering heart of a film that, with its screenfuls of lily-clad Monet lakes and dimly lit Edward Hopper diners, revels in its protagonist's otherworldliness.
"Joy dreams her way through," agrees Henderson, "There's a sense in which she's only half in the real world. Even her clothes are locked in a time that doesn't fit any more. It's nice but it's a quality that people can squash. She wills things to be OK that aren't OK – that's the key to her for me – and that can come across as naive and childlike. I think she has a cut-off point though, where she says, right, that's enough."
Like the moment she suddenly rises up, grabs a weapon and screams "Back off motherfucker!" at the ghost of her ex-suitor, Andy, who has killed himself?
She grins. "Yes, I enjoyed that. I don't like my temper, but I know I have one." She considers. "I think it's good sometimes to have a real old bellow from the gut. If something's bugging you, get it out there."
That fighting spirit, which Henderson has utilised to great effect in roles such as Kelly in Channel 4's Irvine Welsh-penned Wedding Belles, and Kate in the BBC's The Taming of the Shrew, has served her well through her life. After a happy childhood with her two sisters in Fife, during which she can remember being taken to the cinema only once (to see Grease in 1978), stints with local drama clubs gave her the acting bug, and she moved to London to study at the Guildhall School of Music and Drama when she was 17.
"I was quite brave but I was a kid, too, and maybe that helped," she says. "I wasn't frightened of people, but I didn't have a clue about the adult world. My mum says she dropped me off at my hostel by Victoria station then drove away thinking: 'Jesus Christ, what am I doing?' I remember her taking me shopping for mince, potatoes and cornflakes. Meanwhile, my roommate was buying things like natural yoghurt and putting it on this mysterious brown cereal. I didn't even know how to order a meal in a restaurant. But I had my own strengths: I wasn't emotionally or sexually messed-up, I don't think it occurred to me that people even questioned such things. I just knew I wanted to learn, and I would."
She found drama school tough, partly because "they analyse you all the time", and partly because she spent much of her time trying, and failing, to perfect an English accent: "It was like having a golf ball in my mouth." Nevertheless, work came steadily after graduation, first in theatre, then eventually a starring part in the BBC series Hamish Macbeth in 1995. Even so, she says she has never quite shaken off the insecurities that being one of the youngest, greenest kids at college instilled.
"Och yes, I always have wee panics. But that's part of what this job is: you don't know what'll happen next, you think maybe nothing you feel right for will ever come along again. Then when you're rehearsing, you think, I've forgotten how to speak, I sound dreadful, should I pull out? I think Michael Winterbottom helped me a lot, especially with improvising. Wonderland was the first film of his I did, and I'd never done improv like that before. He opened up that kind of madness, that freedom, always asking more of you."
"But also," she pauses, "as I get older I think, well maybe some things will never come my way, and you have to get your head around that. I'm not going to get an Oscar at 30 – that's done. So I'll think of something else."
It's this philosophical contentedness that marks Henderson's life now, happy with her slow-burning, steadily progressing career and her life with her partner "near the beach and the countryside" in Dunfermline. She may still have her "wee panics", but her mid-40s have evidently seen her reach a point of resolution. And that includes a determination not to follow her peers to London or LA.
"You don't have to copy anybody," she says, her chin setting again. "You can sacrifice too much for a job. I spend plenty of time in London and it doesn't scare me, but it's a lonely place, even if you've got friends there. My job takes me all around the world, meeting lots of interesting people. But I think if I couldn't get home, if I couldn't get back to what I consider my real life I'd be frightened. Eventually you have to decide what makes you happy."
At which point she stands up, shakes my hand, and heads off towards the photographer without so much as a quick nip to the bathroom. This is a woman who knows who she is without checking in the mirror.