Lucy Lawless made her name playing Xena: Warrior Princess. Now she is facing jail after a protest on board an Arctic oil drilling ship. So what turned her into an ecowarrior?

Lucy Lawless, cult actor, mother of three and most recently international ecowarrior, makes a point of not planning her life too carefully. Climbing to the top of an oil drilling ship and staying there for three days as part of a protest against Arctic exploration in February was no different. "I thought about it for a couple of minutes and then said 'yes'," she says.

"I never really thought we would get on the boat to be honest. I just thought we'd be stopped at any moment, but it kept not happening. And then there we were. I was totally shellshocked for the first 10 minutes, I just could not believe I was up on top of this thing."

The ship was Shell's Noble Discoverer, which this week left Seattle for Dutch Harbour, Alaska, where it will wait for the sea ice to clear. Then, assuming the US government grants the necessary permits, the drilling begins in what has become a key battleground for environmentalists.

In September Lawless, 44, and seven activists will be sentenced, having pleaded guilty earlier this month to charges of trespass. "Oooh, that sounds so flash, doesn't it?" she says. "Out on bail, baby! But you know, people I've met, oceans campaigners and people backstage at Rio+20, some of these guys were like, 'I've been arrested 30 times.' It was kind of fabulous."

Lawless was in Rio to launch Greenpeace's Save the Arctic campaign alongside Sir Richard Branson, with a trailer narrated by John Hurt and an endorsement from Sir Paul McCartney. The organisation is seeking 1m signatures on a petition it has called the Arctic scroll, calling for a global sanctuary to be established in the vast uninhabited area around the north pole.

But by joining in a direct action and risking a maximum of three years in jail Lawless, who began as a singer but made her name as the leather-clad Xena: Warrior Princess on television in the 1990s, has taken celebrity involvement to a new level.

"We've got to try to keep the momentum going," she says, hours after arriving home in Los Angeles. "Everything's on a need-to-know basis, so you only get three days' warning when something comes up where it might be appropriate for you to help." She giggles. "But in the meantime, I'm just going to encourage people to sign the scroll, in all those places where Xena was really big: Egypt, Brazil, Turkey. Next I'm going to write to the Philippines!"

She says the events of this year have been life-changing. An unlikely action-heroine, nicknamed "Unco" for being unco-ordinated at school, Lawless – who was born Lucille Ryan, and chose her stage name long before she dreamed of trespassing on oil rigs – hated the stunts in her six years as the mythological Xena, who rarely got through an episode without a sword fight. "I was black and blue for about two years, but it's paid off tremendously. You know, if you get enough whacks, your reflexes pick up, and I've become quite good – if somebody throws keys or something, I can catch them."

Her training presumably also helped prepare her for climbing oil ships. She won't talk about the details of how she boarded Shell's ship, and thinks it unwise to speculate about her sentence – "I have no doubt it will be fair, whatever it is, I done it, and fair cop" – but says the whole experience was part fun, part scary, and very uncomfortable.

"Peanuts and chocolate, it sounds like a dream diet – not! – for three and a half days, though we probably could have gone another day."

Someone on the ship played loud music. "Just to be a pain in the arse, to freak you out and unnerve you, but it actually just made us howl with laughter." Shell has denied any involvement in these tactics.

She didn't tell anyone what she was doing beforehand because that would have made them complicit. Her husband Robert Tapert, an American TV and film producer, whom she met on the set of Xena when he was her boss, "had a bit of a shock. I think he thought I was going to chain myself to a bulldozer in front of a tree or something." The task fell to him to tell their two sons, now aged 12 and 10 (Lawless has a daughter from a previous marriage), though she had warned them that if she ever did something like this, it would be because she believed it was important. "They were like, 'yeah yeah, we get it', a bit bored with me, you know? 'Yeah, yeah mum.'" She laughs.

But for all her friendly ease and good humour (Lawless leads a charmed life and she knows it), about climate change she is deadly serious, and her voice rises and changes when she talks about it. "It was interesting to see so many educated people so universally dejected," she says of Rio, "they were obviously completely bummed out and I've never really seen that. Only the politicians were saying [she puts on a pompous deep voice], 'It's pretty good that we came to any kind of consensus', and everybody else was saying, 'No, that's BS [bullshit] and you guys once again have let us down. You know what, the overwhelming message that came out of this was that we're on our own, and governments are rubbish."

Given her views of political leaders, it is no surprise to find that Lawless has no time for those who mock celebrity activists as attention-seekers. "Here's the weird thing. The scientists have been screaming about this for 15 years, and of late, it's unanimous – there's only a few wackos who deny climate change – but nobody listened. So at least with a few celebrities – I call them schlebrities – even though you and I know that's a very silly kind of currency, I think it's all hands to the pump."

Lawless is part of a big family descended from Irish immigrants. Her father was the conservative mayor of Mount Albert, the suburb of Auckland where she grew up, and though she loved and admired her parents for the active part they played in community life, she says what she saw of party politics turned her against it. The family received death threats from anti-apartheid activists when the Springboks played in their town, and people would call and shout abuse if they were angry about rate rises.

But she was brought up with a strong sense of civic duty, and perhaps it is down to her 22 years as the mayor's daughter, as well as later experience with entertainment publicists, that make her such a persuasive spokesperson.

She puts her green awareness down to two events. The first was the ozone hole that developed in the southern hemisphere as a result of CFCs and has led to New Zealand and Australia having the highest levels of skin cancer in the world. The second was New Orleans, where she was filming a made-for-TV movie when Hurricane Katrina hit.

The experience has stayed with her, and sounds horrifying. "The terror of half a million people trying to leave town at the same time, all the roads are gridlocked, people are screaming, and some lone cop is sitting in the middle of a crossroads, there's no radio anymore, your fingers and feet and head are swelling because the air pressure is so low because this massive hurricane is coming. Everybody's trying to get out except the poorest people, who have nowhere to go, don't have any transportation, and they've lived through this before. I saw little black girls walking the other way with their purses, and you know they're going to spend the next two weeks on the second floor of their apartment buildings. That was really a shocking experience. CNN were chasing me down between airports when we finally got out and saying, 'What's going to happen with your movie?' and all you can think is, 'Who gives a shit? Don't you understand what's going on back there?'"

Lawless went back to New Orleans three months later, to give out food at Thanksgiving, and was appalled by the devastation. "You saw people with nothing, who were wonderfully hospitable once upon a time, you know, it was an amazing city. But the reason for going into all that was that I never want to be in that situation again. I don't want my kids to be in that situation where you have an entire metropolis freaking out and it could happen. The predictions are that this will happen."

About 10 years ago Lawless ditched her car, a purple Mercedes with the biggest engine you could buy. "Man, I loved that car." She sniffs. More recently she downsized her house, so that the family now has just a small apartment in Los Angeles as well as their main home in Auckland, where Tapert is filming a remake of The Evil Dead with his lifelong friend and collaborator Sam Raimi, and their boys are at school.

Lawless is currently in LA looking for work. "My plan is to shock people with what I can do, because I've got a few sides to me that I've never used on screen." She has plenty of friends, and has done cameos on the Larry David sitcom Curb Your Enthusiasm, as well as The Simpsons and The X-Files: "I get to do all these really nice, little culty things. It seems to be the world that keeps inviting me back."

Along with most other New Zealanders she was enraged by the French attack on the Greenpeace boat Rainbow Warrior in 1985. That she took the step last year of declaring herself ready to join a direct action is down in part to a friendship formed back then. It was crew member Bunny McDiarmid, now head of Greenpeace New Zealand, who approached Lawless to support a march against plans to allow coal mining in protected forests. The government backed down, and Lawless the protester was born.

Will she join more direct actions?

"I suppose so," she says, sounding more sombre than at any other point in our interview. "If you look at the oil companies' projections up to 2050, their idea of the market share coming from renewables is tiny. It's almost equal parts gas, coal and oil and they clearly have zero interest in doing anything about their business practices."

The hammy voiceover at the start of each episode of Xena ends with the words "her courage will change the world". The connection is irresistible. So did Xena influence the actor who played her? Persuade her she could be a force for good, a fearless fighter against injustice, as playing the Terminator perhaps helped shape the political ambitions of former California governor Arnold Schwarzenegger?

"No, but I knew that would be a very useful hook for people," Lawless says firmly. "I never got myself confused with Xena, trust me. I saw her as a composite put together by so many people. But I knew that she would be useful for this ecowarrior thing." She sounds utterly down to earth now; practical. "We've got to save the Arctic, you know!"