“I HAD a bunch of covers in the States all one week,” says Sarah Michelle Gellar, matter-of-factly. “Somebody said to me: ‘How do you go to the grocery store?’ But I’m the girl who hasn’t washed her hair, and is wearing jeans and a T-shirt. I don’t look like I do in the magazines in the supermarket. That’s not me at all.”

Obviously, this is all very humble and refreshing and down-to-earth. But, to be honest, it’s sheer nonsense.

Gellar looks exactly as she does in the magazines in the supermarket. She’s in heels, some tight brown trousers and a floaty poncho-ish thing that I can’t even name. She is a very beautiful woman. She must know this, and she’s pretending she doesn’t. It’s disarming, and it’s sneaky. The thing is, actors are paid to tell lies and pretend it’s the truth.

Gellar spent seven seasons pretending that she was an angst-wracked vampire slayer, who was pretending to be a normal schoolgirl.

Right now, she’s either being extremely friendly, chatty and forthcoming, or she is pretending that she is. I can’t figure out which.

She’s here to promote her new movie, The Grudge. It’s a remake of the Japanese horror film, Ju-on. Unusually for such a genre, it has the same Japanese director (Takashi Shimizu), a Japanese crew, and is set in Japan. It opened at the top of the US box office, taking $40 million (£22 million) on its first weekend.

I saw a screening two days ago. “What did you think?” asks the star. The way her eyes twinkle, the way she cocks her shiny blonde head, I very nearly believe that she cares.

Terrifying, I tell her, quite honestly. Properly, butt-clenchingly horrible. I’m not great at horror films, but knowing she was in it, I wasn’t too worried. Along with Buffy the Vampire Slayer, Gellar has been in Scream 2, I Know What You Did Last Summer and both Scooby-Doo movies. I was expecting some kind of horror-lite. I was badly mistaken.

The Grudge is an Omen, a Carrie, an Exorcist. The Japanese setting gives it an atmosphere of unsettled alienation throughout, to which the frequent use of subtitles only adds. It might not be the most original horror in the world but Gellar’s performance is polished and restrained and, notably, joke-free. Some fans are going to be in for quite a shock.

“It’s not Buffy,” agrees Gellar. “To leave that show was a huge step. I had everything there.”

So why leave? A desire to do other things? The fear of being irrevocably typecast?

She snorts. Albeit, not unattractively. “If I got typecast for the rest of my life as a strong heroine . . . well, there could be worse things in life to be typecast as. But as an actor, you need new challenges, because otherwise it can get old for you. And if it gets old to me, it’s got to be old to an audience.”

Buffy the Vampire Slayer had a good run. It began in 1997 and kept on stabbing, thumping and generally high-kicking demon butt until 2003. It was funny, sassy and just a little bit sexy. Most of all, it was a witty parody on the endless Beverly Hills 90210 US high school clones. “It’s the metaphor version,” is how Gellar puts it. “High school is demonic and horrible, and we just brought that to life.”

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