At the age of 42, David Thewlis has become unexpectedly rather attractive. Unlike his screen persona - he often plays nervous, edgy types - he seems relaxed and laid back. He sits on a sofa at the Dorchester hotel, casually dressed and sporting an interesting set of jewellery - a silver necklace which has talismans hanging from the chain and silver rings that draw attention to his immensely long, slender fingers. I thought he'd be more manic, like he was in Mike Leigh's 1993 film Naked, the movie that made him a star.

"That was a long time ago," he says. Then he laughs. "But I can do mad," he says. "I am Professor Lupin in the Harry Potter films, remember, and I've just got back from filming The Omen in Prague. That was great fun. I play the journalist who gets his head cut off by a sheet of falling glass." He tells me it's the third time he's had his head cut off. "The time before was in Kingdom of Heaven," he says. "I kept the prosthetic head. Anna," he says, "finds my heads pretty scary but I love them."

The Anna he is referring to is his partner of the past six years, the 29-year-old actress Anna Friel whose biggest claim to fame was that, as Beth Jordache in Brookside, she indulged in the first lesbian soap opera kiss on television. Last summer, their relat ionship was cemented by the arrival of their baby daughter Gracie. As we are talking his mobile rings.

"It's Anna," he says, raising an eyebrow. After he's spoken to her he tells me that she is on a plane back from Prague with Gracie - Friel is also shooting a film there - and that a stewardess has just kicked up a fuss about her changing Gracie's nappy publicly in the aeroplane. "There's no changing area in the loo," says Thewlis. "What do people expect?"

The fact that Friel managed to get pregnant was a minor miracle because she had suffered from endometreosis and was told by doctors she might not be able to conceive.

"Anna always wanted children, so that was a shock," says Thewlis, who has most recently been seen in Basic Instinct 2. "But then she got pregnant and we were delighted. I love being a dad. Gracie is such a good baby. She rarely cries ... I look at her and expect her to change but she hasn't so far so we've been very lucky."

Thewlis obviously loves being a father. "I think it has changed me," he says. "God, I'm happy you know! I eat healthily and I keep myself in trim. I wanted to get fit for when the baby came and I took jobs so that I could be there when the baby was born. I wouldn't have missed that for the world."

Before he met Friel, though, he says he wasn't a happy man. "I didn't really realise it," he says. "I thought I was perfectly fine, really. My career was going along nicely. I'd been in Naked and I had films in the States and I was acting with Marlon Brando in The Island of Dr Moreau. But I had this weird life. I had grown up in a toy shop in Blackpool and then moved to London to do an acting course. I was living in Soho, which is a pretty surreal place to live in, and I was flying all over the world pretending to be people I wasn't."

He met Friel years ago on a plane when they were both doing a push to help the British film industry round the world, along with Jude Law, Rachel Weisz and Emily Watson. They then re-met at a party and have been together ever since. "We both have a northern sense of humour," he says (Friel is from Rochdale), "and we both have a similar attitude to things. We're both neat and tidy although I can see we're going to have to forget that with the baby. Also, neither of us is particularly starry. I get very sick of the paparazzi hanging outside trying to take pictures of us with the baby."

Friel and Thewlis live in Windsor. Their back garden looks on to the Long Walk near the castle where the Queen often rides. "When Elton John got married, a few photographers came and hung out on our doorstep to see if they could get a picture while they were waiting for Elton and David to appear. Anna always looks great in photos. I just don't bother because they tend to cut me out of the picture anyway. But I don't care. I tell the paps that I don't need celebrity to have a career!"

It is true. Thewlis is one of this country's best actors and yet he has been very careful about his career. "I've never been that bothered about doing stage or television," he says. "I just love doing films. With theatre it goes on night after night." The travelling comes at a price though. "There was a time when I was in Australia filming and Anna was in New York, and then she went to Canada and I went somewhere else. We didn't see each other and that was a bad thing for us. It's hard to stay together under those circumstances." Now they try very hard not to be parted.

So how do they work it? Thewlis may say he's adapting his working schedule but he seems to be in demand. He's got another Harry Potter starting soon and he's just been offered something "very interesting" which he says he can't tell me more about but, if he does it, would take him to Portugal. "It's difficult," he says.

But he is determined to fit everything around his family. "Gracie comes with us everywhere," he says. "and we do have a great nanny. Anna was back at work eight weeks after having her." He tells me that Gracie has been to eight different countries since she was born. "We had all those books on how to get a baby into a routine," he says, "but we had to throw them all away. Anyway, what's normal for a baby?"

His own childhood sounds anything but normal. His parents, Alec and Maureen ran a shop. "It was a toy shop during the summer," he says, "and a wallpaper shop in the winter when all the guesthouses were redecorating." He describes Blackpool as a magical place. "I loved growing up there," he says. "I loved the pleasure beach and the arcades and the pier. I loved the influx of people in the summer and the quieter winter months."

His first love was music. He and his friends had a band called Door 66. "I was the singer and the guitarist," he says. They got gigs but two of the boys in the band decided to come to London to be actors. "I thought 'Oh well, I'll go too,' I thought we could keep the band going." All three of them auditioned for the Guildhall and they all got in. "After a while, the other two wanted to leave and go home," says Thewlis, "but I decided to stay.

"I'm in a very good place right now," he says. So what does the future hold for the Thewlis-Friel household? "More films and more babies!" he says. "I just don't know when."