I interviewed Aunt Petunia inside Number 4 Privet Drive

“Here I am, of course we’re not just back in Privet Drive, we’re back in the newly-opened never seen before Privet Drive. I remember it very well but you’re back in a particular moment in Privet Drive,” she smiles.

“I think this really brings me back to the very beginning of the shoot, which was the first few scenes where we began to live in this house. It was so inspirational to walk into this place with all its certificates and wall hangings and photographs of our gorgeous son. It was very inspiring, I remember we were howling when we first got here so then we got very used to it.”

Sitting on a chair in the middle of the room, surrounded by a swarm of Hogwarts acceptance letters suspended on almost invisible threads, it’s difficult not to howl in delight.

Privet Drive is, of course, a very special place in the hearts of lovers of JK Rowling’s tale, and it’s been recreated at Warner Bros. Studios in glorious detail. A short walk through a familiar front door and a homely hallway leads you to a living room that’s leapt straight from the pages of her novels.

From the ornaments and awards on the mantelpiece to the portrait of a beaming Dudley Dursley smiling smugly from a shelf in the corner, everything is here, precisely where it should be.

It’s very clearly Sunday, a day when – much to Uncle Vernon’s dismay – a swathe of acceptance letters from Albus Dumbledore have descended down the chimney, winging their way to Harry Potter ahead of the new school term in September.

And just across the hallway lies that fateful cupboard under the stairs, where the Boy Who Lived spent his formative years.

There’s something missing, though – or someone, rather. Uncle Vernon isn't here, and cousin Dudley is nowhere to be found either.

“I’m obviously very sorry about Richard,” says Shaw of her on-screen husband, Richard Griffiths, who passed away in 2013 at the age of 65. “I was in America and I actually was desolate for days after he was gone because we had had the most wonderful time together here.”

As she settles back into the sofa, the Irish actress shares her fondest memories of the man she created a fictional home with.

“He would regale us with the most marvelous stories, he was a brilliant storyteller and very often the camera could not go to action because Richard was finishing a story,” she laughs. “People were sort of looking at watches and then Richard would say ‘and then what happened…’.”

“He also had the most beautiful handwriting of anybody I’ve ever met. He was trained in calligraphy so his hand-written notes were always beautifully done.”

Shaw’s fondness for her co-star is only matched by her pride in her on-screen son. Harry Melling took on the role of Dudley Dursley at the age of 10, and Shaw says she knew from the moment he set foot on set that he would go on to do great things.

“Harry Melling is absolutely like a plant that I feel we watered,” she says. “When he was 10 he was one of the most talented young actors of that group. He really was the most ambitious – the others were enjoying it but Harry Melling really wanted to be an actor, and his uncle was an actor, and he took it very seriously.”

It was his clear devotion to the craft that inspired Shaw to ring up the National Youth Theatre and rearrange an audition for her on-screen son.

“He actually auditioned for the National Youth Theatre while here,” she explains, “and he missed his audition because he was filming so I rang up the National Youth Theatre and said the reason why this young man is not auditioning for you is because he’s filming and they very kindly set another date.”

Not content with merely rearranging the audition, Shaw was determined to help Melling through it, too.

“I remember this because we practiced it. He did Caliban – all the infections that the sun sucks up – and we rehearsed it here in the gaps and he took off. He later went to drama school and then played my son in Mother Courage at the National Theatre and by then had become this swan of a man. He’s beautiful, talented and I’ve just seen him in the West End being utterly brilliant in Hand to God. He was just astonishing in it. I’m very proud of him.”

The actress predicts Melling – whose grandfather Patrick Troughton is probably best known as the Second Doctor – has a bright future ahead, too.

“It was just wonderful to see a boy start at 10 and now he’s just an excellent actor. Watch that space. He is going to be the Hamlets and everything of the future. He really is a very good actor.”

Being back at 4 Privet Drive doesn’t merely invoke memories of the family she shared a home with, though. The house played host to countless magical scenes and sequences, too.

“I mean some marvellous things happened. There was a car that drove out of Harry’s bedroom. That was a pretty mind-blowing scene to be in because the car did fly and we were up in the bedroom and the car just went,” Shaw recalls.

“But the scene I keep on referring to is in the kitchen, when the owls wouldn’t look at me. They just wouldn’t look at me, they kept on turning their heads right around like that,” she explains, twisting her neck as far as possible, “like something out of The Exorcist because they kept on looking at the camera.”

The film crew’s simple solution to the problem has stayed with the actress to this day. “They hung some mice on my apron so that the owls got absolutely fixated on my apron and looked at me,” she explains, “and I thought, 'there’s something absolutely brilliant about the telling of storytelling, that people go to endless trouble to make something seem like something.' And this set was full of real endeavour by people who were always good humoured, working very hard, a lot of them.”

The magic of the eight-film franchise wasn’t confined to the screen, though. As the Potter phenomenon grew, its stars were propelled to new heights. The series’ stars Daniel Radcliffe, Emma Watson and Rupert Grint became the subject of intense press attention, while Shaw and her family gained their own fair share of notoriety among the fans.

“In Ireland where I’m from, cars would slow down by my father’s house because people would say, ‘that’s Harry Potter’s grandfather!’” Shaw explains.

“Now that says something about the human mind that is creepy. It’s amazing, that people begin to fuse the skin of people who are playing the part with the real thing, so that even my father, who had nothing to do with it, was seen as some connection to Harry Potter.”

Those same fans are known for pouring over the source material and extracting their own theories about Rowling’s characters – and Shaw is no different. She knew nothing of Aunt Petunia’s history when she began filming, but was always cautious about stepping out of the author's planned storyline.

“The only person that was ever told the necessary end of the story was Alan Rickman and he said he literally couldn’t have played it unless he knew, he couldn’t. But I was always terrified that I was going to make a mistake, that somewhere I was somehow going to appear, I was going to become this marvellous witch.

“It was very, very interesting. [Rowling] planned those books brilliantly so that all the nervous fragility of Mrs Dursley is actually psychologically very well founded in the envy of her sister and in the competitiveness with Lily. Maybe it’s sort of like the Queen and Princess Margaret in some way – only one person could really be queen.”

Shaw’s fondness for the oft detested Dursleys is evident: “I was always very pro the Durlseys, I liked who they were, because they’re just trying to do well in life. I think a lot of children have that experience of their parents who they find boring, old-fashioned, nerdy. Actually the poor old Dursleys were just trying to get on with it.”

She does, however, acknowledge that Vernon and Petunia weren’t without their faults.

“Maybe they were slightly trying to top their neighbours, maybe there was a bit of that, certainly Petunia was,” she chuckles. “But that would be an act of imagination, but just a limited imagination, and maybe not a generous imagination. Generosity is the thing the kids learn when they get to the school, so they expand into another sphere. And all they do is they expand their minds, really is what they’re doing, magic is expansion, mind expansion.”

If JK Rowling gave her the opportunity to expand her mind and write the tale of the Dursleys’ post-Potter future, where would Shaw like to find them?

“I think it’d be fantastic to set them in South America or something. That they went to live somewhere quite other, but I don’t think they would.”

Does she imagine that they ever came back to this cosy little living room, with its maroon patterned wallpaper and family memorabilia?

“No. I don’t think they did. I think that’s why all those doors slammed, and doors in cars. They left.”

Why do we all keep coming back, then? Why, nineteen years since the first book hit shelves, fifteen years since the first film opened, and seven years since the credits on the last movie rolled, are people still coming back to Privet Drive, buying theatre tickets for Harry Potter and The Cursed Child, poring over the tiniest details in Fantastic Beasts trailers and immersing themselves in the Harry Potter phenomenon?

“Very basically, it’s very well written,” Shaw states matter-of-factly. “The structure of it is very well written.

“And probably, JK Rowling, who is a very nice person, she’s a very benign spirit. I feel, she has harnessed… she somehow saw the world and saw the opposite of sorrow, saw that there was joy in freedom. She’s captured the cliché of life and twisted it, and freed people by it. I think that’s its power. Its power is that it frees the imagination.”

Warner Bros. Studio Tour London – The Making of Harry Potter unveils the interior set of 4 Privet Drive for the first time from May 27th for a limited period only. For more information see www.wbstudiotour.co.uk