SHIRLEY Henderson is looking for me when I arrive in Edinburgh’s George Hotel. At least I think she is. It takes a minute for me to be totally sure that the figure flitting around the lobby is actually her.

Partly it’s to do with the fact that she has lightened her hair. Partly it’s because she has the build and size of a teenager – and a young one at that. Partly, too, it may be that no-one else seems to notice who she is. And surely they would, wouldn’t they?

It seems not. Even when we finally introduce ourselves and retreat to the café for tea and coffee (she’s buying) and a seat in the window, people pass by and no-one gives her a second look.

And yet Henderson is one of Scotland’s most familiar actors, one who first came to our attention more than two decades ago in Trainspotting on the big screen and Hamish MacBeth on the small one. An actor who is currently in your local cinema in the latest Bridget Jones film and who over the years has tracked back and forth between the arthouse (Kelly Reichert’s distaff western, Meek’s Cutoff for one) and Cineworld favourites (there was that series of kid’s films she was in. What were they called again? Harry Potter or something? You’ve probably not heard of them.)

But here we are unnoticed. Hiding in plain sight as it were. It’s what she does. Henderson is an actor. The rest of it, she prefers to keep to herself. She doesn’t like talking about her personal situation and the shape of her life. She is not, in short, interested in the idea of being a celebrity.

“I like to be left alone,” she tells me at one point. “I’m happy to be anonymous and just to lead my own wee quiet life.”

She must get noticed now and again, though, especially when she turns up in something like the second series of Sally Wainwright’s hugely successful drama, Happy Valley. “A little bit. Not to the point where every second person speaks to you. But there’s a small rise, particularly when it’s on. People want to talk to you in supermarkets or trains. Or they’ll just shout.”

In Happy Valley, Henderson played the whispery, insidious teacher (from Linlithgow as it happens) trying to turn Sarah Lancashire’s grandson against her. This weekend you can catch Henderson at the cinema in Michael Caton-Jones’s new film Urban Hymn, in which she plays a slightly unworldly newbie social worker in a role full of repressed pain and quiet, seething intensity.

On screen, quiet seething intensity is Henderson’s forte. In person she is none of the above. She’s a chatty, self-deprecating 50-year-old who is looking forward to a weekend of gardening and going out to dinner with her friends in Fife where she lives, and where, apart from a few years in London, she has always lived. (Henderson grew up in Kincardine, the eldest of three sisters, and went to college in Kirkcaldy.)

As a movie, Urban Hymn is a strange mixture of mawkish sweetness and gritty realism, like an X-rated episode of Tracey Beaker with a bit of Gareth Malone’s The Choir thrown in, yet set against the backdrop of the London riots of 2011.

But Henderson is as watchable in it as ever and the film does offer an interesting take on the idea of goodness and the collateral damage it can sometimes cause.

“Well, of course,” Henderson says when I bring it up. “You’re good with certain people and you’re bad to people behind the scenes because you’re human. That’s what makes you want to do it. That’s what makes you want to find the character because the best person in the world must have times when they’re tired or sick fed up or just a shadow passes over them the longer they live.”

That idea was one of the reasons she wanted to be part of it. That and the fact that Caton-Jones phoned her and asked her. “I liked him and I liked that he asked. I had one line in his film 20 years earlier – Rob Roy. He speaks his mind. He’s very direct and he just makes me smile.”

I get the impression Henderson enjoys being on set, meeting new people and meeting up with old friends. She’s usually the quiet one in the corner, she says. It was ever thus.

What struck her while making Urban Hymn, she says, is how mature the film’s young actors (including Letitia Wright, who plays one of Henderson’s care home charges) seemed in comparison to how Henderson remembers her own late teens and early 20s.

“They seem capable and comfortable with the whole thing,” she says.

That’s not how she remembers her younger self. And yet.

And yet at 17, Henderson, a girl from a modest enough Scottish background, whose performance history mostly consisted of singing in pubs, decided to go to drama school in London on her own, far from the support of friends and family. That must have required a certain inner steeliness surely?

“I suppose. I wouldn’t have known that language then. We didn’t really talk like that. I would have said to my mum and dad, ‘I’ve heard of this thing called drama school. Can I try for it?’ A simple set of moves. I go for the audition. I get one shot at the audition. I pass it and then get offered a place. And then the next thing wasn’t really about how am I going to deal with London, it was about where are you going to live? We found out about a hostel and that was it.

“So maybe I was confident because I didn’t have a clue. The thought of danger would never enter my head.”

She takes a sip of her coffee and thinks. “Obviously I was maybe a focused young girl and maybe my parents couldn’t stop me because that’s what I wanted to do.”

Anyway, she says, there was little chance of her getting lost in the big city. She didn’t have the money to do that. “I had not a penny. I didn’t know about social life. I didn’t know that’s what people did. It was getting on the underground, going to my wee local Sainsbury’s and getting your food for the week. And that was my life for a very long time until people started saying, ‘Would you like to come out for a drink?'.”

She was, it seems, a very young 17-year-old. "I didn’t know about clothes or fashion or even buying coffee. I had never done stuff like that. So going to sit with college girls and have a coffee was unbelievably new to me. We just didn’t do stuff like that. We went to the York Café in Falkirk. That’s where my mum and dad would take us every so often.”

Well, Falkirk is the big metropolis next to Kincardine, Shirley. “Exactly,” she laughs. “We loved going to Falkirk.

“I was way behind in lots of ways. I had the desire to learn, but I went knowing very little about what other 17-year-old girls probably had been doing. I’d sang in pubs but I’d never sat in one. I’d never had an alcoholic drink. Nothing. I was probably too young, but in some respects I had no baggage because I was so young. I didn’t come with any of that growing up bit yet. That was still to happen.”

Which is fine but it meant there wasn’t a lot of life experience to draw on when it came to the acting. “Blind enthusiasm and not really having a clue in some ways gets you through a lot of stuff,” she suggests. “But you also think: ‘I don’t know anything. I have to start learning. I know nothing about boys and stuff like that.’”

She was shy, too. That aforementioned sitting-in-the-corner thing. Now she says she can blether a bit on set. She’s more at ease in herself. Life will do that for you.

If anything, though, acting is harder in her 50s than it was in her teens, she says. “Maybe it’s the age group. Recently I played a woman who’s got Parkinson’s Disease. It's not out yet." (Kathleen Hepburn’s debut feature, Never Steady, Never Still, in which Henderson co-stars with Théodore Pellerin, has yet to be given a release date.)

"I don’t know if it will be OK," she adds. "But that was hard, hard, hard.

“To get to that point it’s a slightly torturous thing because you’re going so far away from yourself and you’re trying to make yourself do something that’s not natural for your body and make it look natural. That kind of character for me is a great acting role. But actually you want to run from it to begin with.”

In short, she’s a feel-the-fear-and-do-it-anyway kind of actor.

“I find it much harder than I did when I was younger,” she continues, “probably because I recognise how I work now. I recognise what I need to go through to get to it. Before, sometimes you’d just learn it the night before because you didn’t know any other way. I think you just push yourself more now.”

If that’s the case, it works. Henderson has been a regular on our screens ever since the mid-1990s, striking up long-term relationships with the director Michael Winterbottom and with writer Irvine Welsh. Since Trainspotting (1996), she's been in the Welsh-scripted TV drama Wedding Belles and she turned up alongside James McAvoy in the adaptation of Welsh's Filth in which she wears a Cheryl Cole beehive and has phone sex with McAvoy's misogynistic policeman. “Yeah, he wrote that specifically because I’m like that,” she laughs.

Is she in the new Trainspotting? "A wee bit.” (I’m beginning to think “wee" is her favourite word.)

“But I’m not allowed to say anything. I can say I’m in it.”

She pauses, then wonders should she even have said that. She will say she was very happy to work with director Danny Boyle again. “Such a lovely, lovely man.”

Did she realise the first film, in which she played Spud’s girlfriend Gail (if I say soiled sheets … Yeah, that bit), would become quite the phenomenon it has? “It’s hard for me. I was there for three days the first time. I didn’t know what Trainspotting was. I didn’t know what it was going to become. Maybe the guys did because they were all doing very well. But we were in a wee caravan all crammed together and you’d just go in and do your little bit and then you were gone. And I was just so excited that I was going to be in a film.

“For me nothing really came of it but I think in the last handful of years there’s been much more interest in me being in it than there was then.”

Anyway, she says, it’s like the Harry Potter films (in which she plays Moaning Myrtle) or the Bridget Jones movies (as Bridget's friend, Jude). She loves that people love them but the fact is the fans know much more about these films than she does. “They’re watching it so many times. Me, I’ve done the film and I’m moving onto the next film.”

Still, she adds: “I get a wee tickle when people still love them.”

When it comes to scripts, does she say yes more often than no? “Probably, yeah. I don’t get as much as you might think. It’s not like every single week I get three or four scripts coming in. I couldn’t cope with that. I don’t want to be inundated. It would drive me crazy.”

And yet she has just finished bits and pieces in four different projects including Trainspotting (don’t tell anyone), another Michael Winterbottom film and a film by the South Korean director Joon-Ho Bong that saw her up a South Korean mountain surrounded by thousands of people on the first day of shooting. She is busy.

Is she still ambitious? “It’s a word I don’t really understand any more. A lot of people ask me: ‘What’s the plan?’ I don’t have a clue really. I think I still have a drive that annoys me sometimes. I want to go, ‘Stop, just take a moment.’ There are days when I think, ‘This is so hard and I’m so tired and I’ve forgotten where I’m living.' I could easily stop. But then something pulls you back and you have such a great time with people. That’s what it is. It’s not meant to be easy. There might be moments of easy. For some folks it is but for me it never has been.”

And when she does stop, when she has time to herself, what does she do? “Just go into my wee garden. I’ve got a tiny wee garden. I’m not big into it but I could be if I stopped working. You have to be there every day. But I’ve got tomatoes growing in hanging baskets. I know that’s really boring but I’m thrilled and I’ve eaten some of them and they’re really nice.

“I like gardening. I do. I’m not very brilliant at it but I love buying flowers, I love garden centres.”

So, Dobbies, I start to say … “That’s my favourite place.”

Urban Hymn is now on limited release