HOLLYWOOD stars are well rehearsed in seeming to be modest about their Oscar chances. Not Catherine O'Hara.

Despite being tipped as a long shot for nomination after the National Board of Review named her best supporting actress for her performance as an actress playing a dying Deep South Jewish matriarch in Christopher Guest's For Your Consideration, a parody of the hysteria surrounding the Academy Awards, she refused to play the game, dismissing the speculation as a bad case of life presuming to imitate art.

"No, no, it's not about me, " she sighs, mimicking her character Marilyn Hack's coy response to media speculation about her being an Oscar contender for a performance she was still filming. "It's all about serving the script, serving the word, serving the director. I serve my character, it's not me, it's not me, it's not me!"

O'Hara sips a cup of tea. "But of course it's all about her, " she says.

She grimaces suddenly. It turns out she put saccharine in her tea instead of sugar. "I was fooled by the white packet. In the US the sweeteners are in blue or red and the sugar in white. I put the fake in.

That tiny amount and it's so sweet!" She goes to the bathroom of her suite in the Dorchester and empties out the cup. "Think of the chemicals!" she says.

O'Hara doesn't do fake. Much of her career has been spent sending up actresses. She made her name at the Second City Theatre in Toronto in the 1970s with Dan Ackroyd, John Candy, Eugene Levy and Gilda Radner writing and performing sketches for a television series about a low-budget television station called SCTV.

Among her targets were Katharine Hepburn, Elizabeth Taylor, Monica Vitti, Jane Fonda and Liv Ullmann. So suddenly finding herself getting celebrity treatment as a possible Oscar contender came as a bothersome shock. "I had to answer questions about it. There's no way not to sound stupid or wanting or yearning. It's pretty ugly really. And I'm even embarrassed that I'm embarrassed."

It's not that she doesn't enjoy the Oscars. "You watch all the faces of the nominees, with the cameras in their faces, and you're trying to figure out who's not going to get called and how they'll react when they're not, " she says. "It's a cruel world and this is just another form of cruelty. But it's fun too, you can't help yourself."

She's been through it all four times with her production-designer husband Bo Welch when he was nominated first for The Birdcage and then for The Little Princess, Men in Black and Batman Returns.

"As soon as he was nominated - although I swear to you he definitely deserved to win - he'd say right away, I'm not going to win.

He knows the way it works. By the time it came, he was still cool but people got through to him. Your friends and family will not say, 'Oh it's nice but you're not going to win.' It's, 'Why wouldn't you win, you're the best?' And he's fighting that the whole time."

O'Hara featured in the awards ceremony three years ago. "A song from Christopher Guest's folk music spoof A Mighty Wind got nominated and Eugene Levy and I got to perform it in character. So as it wasn't us and we hadn't even written the song, there was no pressure on us. But the real fun was to hang around the rehearsals. They have these actors there all day for so many days before the Oscars. It's a TV show so they have to rehearse every possible move. By the time the Oscars come they've rehearsed every possible winner and they have different actors to play these winners.

"And they don't just come up and do the movements, they go on stage and they do speeches and they're doing the speeches that those winners would do. I wish I'd a tape of it. If there's a red-haired nominee they get a red-haired actress. They get similarly coloured people depending on what colour a nominee is. You wondered what would happen if any of these stand-ins got so into their performances that they actually showed up on the night and tried to be the stars they played."

Christopher Guest, the son of an English peer and an American mother - he's now a peer himself, and gave his wife Jamie Lee Curtis a thrill one day by taking his seat in the House of Lords - doesn't believe in rehearsal. All his movies from the Rob Reiner-directed This Is Spinal Tap to Waiting For Guffman and Best in Show to A Mighty Wind and now For Your Consideration have been improvised. "You just show up and the camera rolls, " says O'Hara. "From take to take he'll say pick up on where you were talking about so-and-so, but it's not the words, you could still tell it in a completely different way, it's just the beat of the story he needs to get. There's a scene outline and each scene has story beats, so he'll make sure he gets that, but otherwise he says no to nothing."

It helps that almost everyone in his movies also writes. O'Hara wrote all her own sketches on SCTV. But she admits to being scared when she first teamed up with Guest for his theatre spoof Waiting For Guffman. "I thought I had to come up with something great every second, but Christopher pointed out that listening is just as important. It's like you doing this interview with me.

You're being forced to listen to me, so if we were improvising this you'd be listening. You wouldn't be going, 'Oh I better think of a joke here.'

"So with Christopher you just relax and try to be in the moment and you try to be the character and he cuts it down to what it is.

But there are reams and reams of footage that's not necessarily funny or doesn't help tell the story that he leaves out.

"Christopher says very few people can do this sort of acting. I don't want him to know but I think everybody can do it. I'm sure people in your family like in mine are funny. When you're relaxing with people you love and you're safe with them, you're free to be your best and I think being your best is also being your funniest."

Born in Toronto in 1954, O'Hara has both Canadian and US passports. "I got American citizenship three ways. I applied on special merit grounds, which means you try to persuade Americans that they need your talents. That's what I already had in the works and then I got married to an American. And after all that I won the citizenship lottery."

She's Irish on both sides through her great-great-grandparents.

"My family names are Shart and Kelly and Meehan and O'Hara. I even have a sister called Maureen O'Hara, which confuses everyone in Hollywood. But none of that would have qualified me to be Irish. I think you have to have an Irish grandfather."

She is a brilliant impersonator - her voice can change into anyone mid-sentence and she regularly does cartoon voiceovers (she was DJ's mother in Monster House and Penny the Porcupine in Over The Hedge) - but her only serious dramatic role was playing a staunch republican mother in 1970s Dublin in The Last Of The High Kings, an adaptation of Ferdia Mac Anna's novel.

She's been in all Guest's movies, but she's no idea if she'll be in another. "He'll always say at the end of a movie, I don't know, I think this might be the last. He won't commit to anything until he's sure of what he wants to do. When he does, Castlerock and Warner Brothers let him go into pre-production and he and Eugene start writing. They don't call any of us until there's a script.

"Chris is a very private man. I don't really know how his mind works. But I know if he comes up with something that makes him happy, he'll call. And I'm always waiting for that call."

'For Your Consideration' opens this Friday