The actress – best known as Narcissa Malfoy in the Harry Potter series – tells us about her latest adventure with Doctor Who…

Was it exciting to be asked to appear in Doctor Who?
“Well, actually I was LA when the recent Doctor Who phenomenon started so I sort of missed it. I remember I was with David Tennant and Billie Piper at the theatre a while back and as we left there were all these people on the street descending on them. I had no idea it had become this huge deal. And then they asked me and I sat down with a whole load of tapes and thought ‘Oh, great – yes – I’d love to do this’.”

You weren’t a fan in your youth?
“I grew up in Africa so I didn’t have all that hiding-behind-your-sofa thing. I was actually quite alarmed when I saw the script at how scary they are allowed to be because it’s a children’s show.”

Your episode, The Vampires Of Venice, is set in 17th century Venice – what can you tell us about your character Rosanna?
“We find her as a matriarch running a school for girls at the time when The Plague has hit Venice. The school is for young women, seen as a haven of safety and disease-free for these young girls brought into it. As The Doctor arrives he begins to see why they’re being taken into this school and my evil master-plan!”

So,she’s an enemy of The Doctor?
“There is actually a huge sympathy between the Doctor and her. They’re both travellers and they’re both out of place where they are in the world.”

Is there a connection between them because Vampires, like the Doctor, have lived very long lives?
“I have to be careful what I say. But… yes… exactly, that’s the connection they have.”

Vampires are very fashionable right now, with Twilight and True Blood…
“You’re nobody in this business unless you’ve got a pair of fangs in at the moment! It was my turn.”

Have you finished filming the final Harry Potter film yet?
“Not quite. I don’t know if I’m going to be called in again but there’s a few more months of filming. They have been filming these huge great battle scenes at the very end of the film, where Harry meets Voldermort. That takes weeks and weeks and weeks to film because there are hundreds of extras, and 48 main cast involved, and everyone in make-up. It takes a long time!”

Have you been invited to the theme park yet?
“No I haven’t but I think it sounds fantastic. The art department on the films are brilliant. Some of the best artists in Europe work on those films, and I know they’re taking a lot of the props from the film to the park. I think that’s great because it’s 12 years of people’s hard work – it should be displayed for people to see.”

Are the Potter fans quite obsessive?
“I never, ever get recognised by Harry Potter fans. I look very different from my character – they age me, and I have the blonde hair. Even at the premiere – people don’t connect me with her.”

*Helen McCrory stars in Doctor Who on Saturday May 8 at 6pm on BBC1*