Glasgow-born Kelly Macdonald, 43, began her career as Diane in Trainspotting. Her film credits since have included Gosford Park, No Country for Old Men and the voice of Merida in Disney’s Brave. She has two children with her ex-husband, Travis bassist Dougie Payne. On TV, she has appeared in State of Play, Boardwalk Empire and now stars in BBC1 thriller The Victim which starts tomorrow at 9pm and continues for the next three nights.

In The Victim you star as a grieving mother who, 15 years after her son was murdered, reveals the new identity of his killer. What drew you to the role?

She felt real: sympathetic at times, unlikable at others. The title is great because you’re wondering who the victim is. Reading the script, my allegiances were switching, so I hope it’s the same for viewers.

As a mother yourself, how did it feel going to such dark places?

It wasn’t so much as a parent, just as a human. Those emotions apply to anyone. Mothers don’t have a monopoly on them.

It will inevitably remind viewers of the James Bulger case. Were you sensitive to that?

I was very aware of the James Bulger tragedy at the time because it was so nightmarish. I can see why people might compare, but The Victim isn’t based on any specific real-life case. There are parallels but it’s a fictional story. I don’t work like that anyway. I have played real-life figures like Olive the nanny in Goodbye Christopher Robin, but tend not to base parts on anyone in particular. I’m always impressed by actors who create character dossiers and do lots of research. I’m more instinctual.

It’s set in Edinburgh. Does it show off Scotland well?

Scotland looks stunning, which I’m really proud of. Our director of photography was Swedish and it does have that Scandi feel. Scotland’s quite Scandinavian anyway – we’re all related, all Vikings.

You’ve just filmed new Brit-Japanese thriller series Giri/Haji, which the BBC says is “unlike anything seen before on UK TV”. How so?

It reminded me of Trainspotting – groundbreaking, with a tangible sense of excitement about it. Because Trainspotting was my first job, I thought they’d all be like that. Turns out that’s not quite the case, sadly, but Giri/Haji truly was. The crew ended up nicknaming it “Gary”, so I called it that too.

Take us back to Trainspotting. How did you come to be cast?

They wanted to discover a fresh face to play Diane, so gave out leaflets and I was lucky enough to get it. It changed my life, but I’m glad it happened over 20 years ago. Nowadays, with smartphones and social media, a new actress in a role like that would have a different experience of overnight fame. I was able to build my career gradually. Today it would be much more extreme and harder to cope with.

How was the sequel, T2 Trainspotting, a couple of years go?

Lots of fun, like a school reunion. Diane had a fairly minor role, which was great because I could just enjoy it with no pressure.

After T2 came the TV adaptation of Ian McEwan’s The Child in Time – another harrowing story about a grieving mother…

Don’t worry, I’m not getting typecast. You reach a certain age and get cast as a mother. And because it’s dramatic, sometimes a grieving mother. Plus of course, I give good grief face! Benedict Cumberbatch was amazing to work opposite.

Do children recognise your voice from the Disney film Brave?

Sometimes I get introduced to little girls as the voice of Princess Merida, and you can see it clicking into place for them: “Aha!” She’s a heroine for girls, which is cool. I have sons, so they’re into different stuff I’ve done. One day on “Gary”, I was waving a gun around nonstop and went home with an aching arm. I said: “I bet not many mummies come home complaining of a sore gun arm.”

Helena Ravenclaw was the final role to be cast in the final Harry Potter film, meaning you’re the last person ever to join the Potter franchise…

Really? I knew I was last to film my scenes, so I was pretty much acting on my own – just me, a green screen and a cardboard cut-out of Daniel Radcliffe. It’s always a thrill when I go into the Harry Potter shop and see the navy-and-grey scarves. I think: ‘That’s my house!’ I don’t support any sports teams so that’s as close as I get. Team Ravenclaw!

You’ve described The Victim as “almost Shakespearean”. Fancy doing some actual Shakespeare?

I’ve gone for a couple of parts but didn’t get them. Racking my brains, I’ve never done any Shakespeare. That’s probably illegal for a British actor! Then again, it’s compulsory for British actors to do Harry Potter. At least I’ve ticked that box.

You recently played Mrs Hudson opposite Will Ferrell and John C Reilly in Holmes & Watson. Would you like to do more comedy?

I got to improvise, crack jokes and fanny about, basically. You can’t beat a bit of fannying about! I’d love to do more but I tend to get these dark, troubled roles. Maybe it’s my face again.

How did you feel when your old school, Eastwood high in East Renfrewshire, named its theatre and drama centre after you?

Really honoured. They didn’t have anything like that in my day, so it’s great they do now. I got to pull a little rope at the opening and talk to the drama students.

You’ve won an Emmy and several Screen Actors Guild awards. Where do you keep them?

On a high shelf in the sitting room. I noticed recently that the inscription on one of the SAG awards for Boardwalk Empire is wrong. It’s supposed to be for ensemble in a drama series but it says comedy series. I got in touch with my castmates but theirs were all correct. Maybe they were trying to tell me something!