Actor and writer Julian Fellowes was determined to "do a Hitchcock" and appear in the first film he's directed, Separate Lies.

Unlike Alfred Hitchcock, he didn't do it in the flesh - he's only seen on a TV screen.

A scene from the now-axed BBC1 Sunday night series Monarch Of The Glen, in which he was Lord Angus Kilwillie, is playing on a TV set in the background at one point.

Fellowes was thrust into the spotlight as a writer after winning an Oscar for the Robert Altman-directed British drama Gosford Park in 2001. Before that he spent years as an actor, often in upper class or authority roles. His CV is dotted with characters called Lord, Sir, Colonel and Major.

But he does have some Northern roots as his grandmother's family came from the North-East. "I know Darlington very well, my grandmother came from Neasham. I went to Ampleforth School and my first girlfriend was from Barnard Castle," he says. "I love Yorkshire. I think it's rather misrepresented when you see those grumpy Northerners."

Separate Lies, which he adapted from Nigel Balchin's novel, A Way Through The Wood, involves adultery and murder among the upper middle classes. Tom Wilkinson, Emily Watson and Rupert Everett take the leading roles.

He liked the unclear morality of the tale, which he regards as a strength of European cinema. British cinema, on the other hand, usually demands that the bad guys are dealt with.

"Normal life is not a gangster movie where people are doing bad things and keeping guns under the bed. Most people aren't terrible but may have done something terrible. I like films where you change your mind about characters and think, 'I'm on her side, and then you're not'," he says.

Fellowes wrote the screenplay before winning his Oscar, as one of many scripts he was writing and which weren't getting made. "I thought that being an adaptation, it had more chance of being taken seriously because people trust adaptations more than originals. It's a great deal harder to get an original screenplay made," he says.

The power of an Academy Award shouldn't be underestimated, he feels. "It's the nearest thing to a legend that the 20th century created," he says. "When you've won an Oscar, it's almost as if you're carrying around the bones of a medieval saint. I have other awards but people don't say, 'can I hold it? Can I touch it? Can I be photographed with it?'. It had that effect, at least for me.

"It's different if you are Ronald Harwood winning for The Pianist because he was already a very successful writer. I had that king for a day thing where no-one had heard of me and suddenly I stepped out of the cupboard holding an Oscar."

Before Separate Lies he'd had experience of film-making through a company he ran with director Andrew Morgan. They made several children's series for the BBC, including The Prince And The Pauper and Little Lord Fauntleroy.

"Technically, I was a writer and he was a director. The truth is we made them together. In that sense I had a fairly strong training to what was going to happen," says Fellowes.

"When you go to directing, you bring different strengths and weaknesses. I had a lot to learn about post-production. But I knew how the set worked and that you have to make 50 decisions at once and think about the scene on the carousel you're not shooting for two weeks. None of that came as a surprise to me. I would love to do it again."

At least Separate Lies was released in both the US and the UK, something that two-thirds of British-made movies don't achieve. Critical response was good, too, with several award nominations. "All of that is pretty rewarding," he says. "I think it's a film essentially aimed at adults, for people over 30. People under 30 would not know what the hell it's about."

Before he directs again, he has plenty to keep him busy. He wrote the book for the stage version of Mary Poppins, which has been running in London for a year and is transferring to Broadway in the autumn. He's had a novel published but hasn't found time for a second one yet.

"This is my moment and, like everyone else, it won't last forever and I don't want to faff around when the film industry is interested in me. You have to respond to what's happening."

What hasn't happened is a move to LA in the wake of his Oscar success. He hasn't whisked wife Emma, a royal lady-in-waiting, and son Peregrine off to the States. "It's like when you write your first novel, you should write about what you know and maybe spread around later. For anyone creative to move right away from their roots is a little bit tricky.

"I'm not an anti-LA person. I've always had a jolly good time there and have a lot of old friends there. But I'm English and my wife is English and my son is English. You have to raise your child in the culture you want it to feel at home in."

Separate Lies is available to buy and rent on DVD from Twentieth Century Fox Home Entertainment from Monday.