Alyson Hannigan: Bedknobs and broomsticks

It's a drizzly, grey day in London, but Alyson Hannigan is doing her best to brighten things up with her thousand-Watt film-star smile. The actress made her name playing Willow in all seven seasons of the phenomenally successful TV series Buffy the Vampire Slayer, a role that involved her transformation from sweet-natured computer geek to vengeful flying witch, by way of becoming a lesbian in love. She also appeared in the American Pie movies, a trilogy of bad-taste teen sex comedies that culminated in 2003's American Wedding, in which Hannigan's character, Michelle, finally got her guy up the aisle - and the actress got star billing.

Now, she's in London for the stage adaptation of the Hollywood rom-com When Harry Met Sally, directed by Loveday Ingram, with swinging new tunes by the jazz wunderkind Jamie Cullum and his brother Ben. Rob Reiner's movie, which was written by Nora Ephron and starred Meg Ryan and Billy Crystal, was a huge hit when it appeared in 1989. Now Hannigan, along with her co-star, Luke Perry, best known for Aaron Spelling's glossy school soap Beverly Hills 90210, is hoping to tug our heartstrings all over again.

If today's performance is anything to go by, it's a task that Hannigan is more than equal to. Slender and huge-eyed, with long, glossy red tresses and an infectious giggle, she has obviously already won the affection of all the staff at the Theatre Royal Haymarket, where the company is preparing for its first technical rehearsal. She's bright, chatty and beguiling, but occasionally her naïve enthusiasm fails to be entirely convincing. Everything, it seems, is "cool" or "soooo cute" - the London streets, the quaint bayonet-type light bulbs around her dressing-room mirror ("We don't have those in America, only the screw kind") and the crackly speaker-system, through which voices float from the stage. But then, for an actress who has lived and worked in highly polished Hollywood since childhood, the shabby backstage areas of a West End theatre must be something of a novelty.

Hannigan was born in 1974. An only child, she spent her early years in Atlanta, Georgia, where through the connections of her photographer father she started modelling and acting in commercials. Even then, she says she had her future career planned. "I just knew I wanted to act," she says, settling her tiny frame into a tatty armchair. "That and soccer were my two loves. But acting has always been my passion, and I just wouldn't have known what else to do." So determined was she, in fact, that at the age of 11, with the help of her parents, who were by then separated, she set up meetings with various LA agents. Their response was positive, so she and her mother waved goodbye to Atlanta and decamped to Hollywood. There, she went to North Hollywood High School, took a small role in the film My Stepmother Was an Alien at the age of 13 and worked steadily in TV. And then, in 1997, along came Buffy.

Hannigan was hired as Willow without ever having actually seen a script - a cock-up on the part of her "not so good" agent of the time that was, she now says, a blessing in disguise. "If I had seen one, I would have wanted the part so badly that I wouldn't have got it," she explains. "'Cause, y'know, you can psyche yourself out." In fact, as she puts it, the scripts "were phenomenal and they just kept getting better and better. The response from the critics was awesome, the fans embraced us - and the rest is history." I put it to her that some Buffy fans are rather more enthusiastic about the show than is strictly healthy, harbouring a fascination with its stars that borders on the obsessive. "No, I don't think they're obsessive, they're just dedicated," she corrects me carefully. What about the girl whose admiration for Hannigan was so great that she personally delivered the gift of a live horse to her home? "She was really, really dedicated."

Asked why she thinks Buffy inspires such devotion, she seems to be stumped. "I dunno. If I could figure it out, I could make a lot of money developing shows." Not that she's doing too badly anyhow. The Buffy series became so huge that its stars were raking in a reported £160,000 per episode in the final season, and American Wedding netted her £1.8m. "It's nice to do what I love to do and make a nice living at it, and to have the luxury of being able to choose jobs, rather than having to take them to pay the rent," she admits. "That's something I'll always be grateful for. But it's not why I got into the business."

While the character of Willow ran the emotional gamut, Michelle from the American Pie movies is sheer screwball comedy. Initially a nerdish flautist whose every anecdote begins with the phrase: "This one time, at band camp", she later emerges as a saucy sex kitten who has found a kinky and far from musical use for her instrument. Hannigan's performance in the role is a riot. "I was attracted to the character partly because of the shock value," she admits, eyes twinkling, "but mainly I just thought she would be so much fun to play." Possibly because of her on-screen antics, some of Britain's tabloids have published racy tales of Hannigan's own sexual exploits - stories that report her coupling on a pool table, in a car while driving and even on a Ferris wheel. True? "Eeeugh. I don't know what you're talking about," she says, looking genuinely astounded. "No way! No, no, no, that's not true!"

Still, she will be giving theatre audiences her best fake orgasm in When Harry Met Sally's most famous scene. The show marks her stage debut, and she says she's hugely excited about opening night - but she's also full of apprehension. "I'm sure I'll throw up on the day," she says. "There have been a couple of times in rehearsal when if you'd said to me, would I like to go home, then I would have said: yeah! But at least I don't have to worry about whether people will like the piece. It's such great material, my job is just not to screw it up."

Hannigan says that, as a teenager when the film first came out, she dreamt of playing Sally. "I was so jealous that Meg Ryan got to play her. The film is just fantastic across the board - the writing, the acting, the directing. Every single person can relate to something in that movie; it just hits so close to home - particularly about the differences between men and women. It's just a wonderful, wonderful film - touching and funny and very romantic." Is she a romantic? "Yeah, I really love love," she says, grinning, her entire body visibly relaxing and her face more open and full of real feeling than at any other moment in the interview. She married her fellow Buffy actor Alexis Denisof (he played Wesley Wyndam-Pryce, the upper-class English twit with an occult bent and, yes, a heart of gold) last October, and is missing him terribly. "I'm a newlywed and I've left my husband across the pond," she moans, mock-tragically. It is Denisof, she says, who "talks me down off the ledge" in long-distance phone calls when she has had a particularly tough working day. "And he's coming out next week, so I'm very happy," she adds, her eyes bright and dancing like a honeymooner's.

So, what's next for this all-American sweetheart? She has struck a development deal with NBC, and says that in the immediate future she's planning to stick with comedy, "because I just love it so much", rather than tackling anything more heavyweight. Right now, she is focusing on doing justice to Sally. Previous Hollywood visitors to the London stage have generally failed to find favour with the British reviewers. Is she nervous about the possibility of suffering a critical mauling?

"I've heard so much about how brutal the press here can be," she winces, "but I'm doing this for the love of the piece and for the experience, so I'm going to try not to care. I just hope there's a good quote that the theatre can put out front. Y'know, you walk past theatres and there's all these posters saying 'blah blah blah' " - she adopts a plummy faux-English tone - "but occasionally you walk past one and all it'll say is, 'Good'." She chuckles. "And you're like, hmmmm, that's just one word. I wonder what the surrounding words were. 'This is so not good'! So I'd like to have at least one nice whole sentence, just for the poster. That's my goal."

'When Harry Met Sally' is previewing now and opens tomorrow at the Theatre Royal Haymarket, London WC2 (0870 901 3356)