OTTAWA - The $20-million war film Passchendaele - the most expensive movie made in Canada - was written and directed by Paul Gross, who also stars in the film and who also wrote the song that's played over the final credits.

"He even wanted to play my part, but I said, 'Paul, I think you should let go,'" laughs Caroline Dhavernas, the 30-year-old Quebec actress who co-stars as Sarah, the pretty nurse who falls in love with Gross's First World War soldier.

Passchendaele is a bold experiment in Canadian cinema for its cost and its scope, and it could also be a breakthrough for Dhavernas. She has had small roles in big Hollywood films - as the Girl Friday to private detective Adrien Brody in Hollywoodland, for instance - and big roles in smaller Canadian films - Niagara Motel, Surviving My Mother, and many others. She is probably most familiar for her TV work: she was Marilyn Bell in a TV movie about the young swimmer and she played the lead in Wonderfalls, a short-lived Fox series, as a woman who works in a Niagara Falls gift shop and communicates with inanimate objects.

The epic Canadian war film is something new.

"I've never heard of Passchendaele before, so I think that's one of the reasons it's so important to tell this story," Dhavernas said in Ottawa recently, on the eve of a gala that launched a countrywide publicity tour for the film. The film opens Oct. 17.

"Because most people don't even know about it. We do know more about the Second World War than the first. It was a big victory for this country and I think it's the story of the sacrifice of these men and women who gave everything for this cause. To me that's what the heart of the movie is and I think that's why people respond to that aspect.

"I've never been very pro-war - not at all actually - and I'm always trying to find a reason to find something human in it. I think that's what it is: It's the camaraderie between the boys and the women who were there to serve as well, nurses, that's what's beautiful about war. It's what happens between men and women who were there."

Her character is a damaged woman who meets Gross's soldier when he returns to Calgary with post-traumatic stress disorder. They fall in love, and then find themselves sent overseas. They have a fateful reunion in Passchendaele, at the Belgian battlefield where thousands fought and died in the mud and squalor of trench warfare. It was a battle that helped define the country and made Canadian troops legendary for their courage and fighting skill.

For Dhavernas, though, the initial attraction was the script's love story.

"Usually very epic female characters to me are the same and I think in this one there's an edge that's very interesting . She's just very broken and so is he and they find each other in this time, in their country, when the most horrible atrocities are happening, and give each other the strength to get through it. And I think they're both completely broken and there's this chemistry that comes from that place where they understand each other and what they've been through."

Shooting the battle scenes, in fields near Calgary, gave Dhavernas a taste of what the real thing must have been like, even though she says the actors only experience "a tenth of a millionth of what it was." But standing in the cold water of muddy trenches, she says, provides an echo of what the real thing must have been like.

"That strikes you when you least expect it. In the middle of a scene sometimes. When you're feeling everything, that you remember people went through that for real. And still do. Right now."

Dhavernas is the daughter of Quebec actors Sebastien Dhavernas - a Liberal candidate in the Outremont riding in the federal election - and Michele Deslauriers. She started in the business when she was eight years old, dubbing Babar cartoons, and got her first movie role when she was 11, in the 1990 film Comme un voleur.

It's not unusual for her to go on publicity tours, but most of them have been through Quebec by bus. The cross-Canada tour by plane is both new and welcome, because she's been in other Canadian films that don't have the money for such promotion, and often fail because people just don't hear about them.

"When it falls at the last step of publicity it's always so heartbreaking for me because for me it's just a couple of months of my life but for the director - mostly with directors who write the movies - it's years of their lives," she says. "It's a crucial part of making a movie.

"For once we have the budget for a Canadian film and the interest, because it's the pride of the nation, it's a moment in history that we celebrate, it's Paul: He's a huge star in Canada."

Passchendaele is the first time Dhavernas worked with a co-star who was also the director, and she says she was impressed with Gross's ability to focus on acting.

"I was impressed by the amount of energy had," she says. "There wasn't a minute when he had nothing to do."

There just wasn't time for him to play the nurse.