He appeared in a loincloth for Greystoke. Now Christopher Lambert slips on a cassock to tell the tale of a Polish priest murdered by his government. Paul Mansfield talks to the French actor who's changing direction with every movie he makes.

The star has just been knocked flat on his back, for the fourth time this morning. In a decrepit gymnasium with a boxing ring, supposedly in Warsaw - actually in Lille in northern France - Christopher Lambert, Europe's leading man, is pursuing an older boxer round the ring, lashing out at him with more enthusiasm than skill. When the other man's back is turned, Lambert swings a final punch - and is promptly floored for his trouble. It's an arrogant, petulant display; and what's especially unusual is that Lambert is playing the part of a priest.

No ordinary priest either. Father Jerzy Popieluszko was the Polish priest murdered by the secret policemen in 1984. With Lech Walesa, he was a leading light of Poland's Solidarity movement; young and charismatic, outspoken in his criticism of Poland's communist government. His death made him a martyr in the cause of the banned trade union: his grave in his old parish churchyard in north Warsaw is a place of pilgrimage for ordinary Poles and visiting dignitaries alike. To Kill a Priest, directed by the exiled Polish director Agnieszka Holland, tells the true story of Popieluszko's murder.

The part is a deliberate new direction for Christopher Lambert, a star in his native France for Luc Besson's cult movie Subway, and well known internationally for Greystoke: Legend of the Apes and Highlander. He professes a close interest in real-life events in Poland, but a natural disdain for wider political issues. "I'm not political at all. I find politics amusing, really. Instead of playing Monopoly you could have a game called Politics. That's all it is, a big game."

We are in his trailer for lunch, which - this being France - is a baguette from the boulangerie across the street, washed down with a bottle of Heineken. Lambert is a powerfully built man of 31, whose intense eyes and brooding good looks give him an almost intimidating air - offset in his case by a broad grin and a pair of scholarly white spectacles perched on the end of his nose. He's a thoughtful man, who gives the impression of being completely absorbed in whatever it is he's doing at the time. So the talk begins, naturally, with Poland and its politics, Father Popieluszko, the current film.

"This isn't a political movie in any obvious way," says Lambert. "We see the state of emergency and war in Poland, but what's at the forefront is the story of one man trying to fight against a government, any government."

To Kill a Priest has Ed Harris (John Glenn in The Right Stuff) as a fanatical police captain consumed with hatred of Lambert's Solidarity chaplain. But the priest, says Lambert, had other, equally powerful enemies. "He died because he upset too many people. Everyone in Poland works hand in hand not to make any trouble. The church and the state cooperate with each other all the time, and they treat any kind of disruption as a threat. If the captain didn't kill the priest, who knows? Perhaps he would have died in a different way. They always die, that type of character."

Lambert researched the part by watching videos of Popieluszko (pronounced "Poppy-awush-ka") at mass. "He was an amazing preacher. Sometimes he's have 40,000 people at one service, and their emotions would be at fever pitch. When you watch those tapes it's almost like watching a rock concert."

It was this combination of the religious and the worldly that attracted Lambert to the part. Until recently he has been in danger of becoming Europe's latest sex symbol - a fate of which he's all too well aware. "You should never try to destroy the image people have of you," he says. "But you can change their perception. That's what I'm trying to do with the character of this priest. You can look at a priest as a sex symbol if you want to - and with this priest some people probably did - but it's not automatic."

Lambert is bilingual, but Hollywood has decreed that his French accent is too strong, and he now has a voice coach to make it more "mid-Atlantic". "But one thing I will never be," he says with a grin, "is the standard New York or Californian leading man. That's just not possible."

In career terms, he is currently on the threshold of an enormous leap from local hero to international star. It's a prospect which hardly fills him with awe. "It's luck, and I'm just trying to respect that luck," is his comment. The son of a United Nations diplomat, he's refreshingly level-headed about Hollywood and its inhabitants. "You can't trust those guys at all. They'll tell you any kind of crap just to keep you happy. On Greystoke, my first big movie, that was the first time I learned. 'Keep the actor happy!...' Everyone was saying it! Keep the actor happy! What does it mean?!...

"So I trust the audience. Because they can look at you and say, 'I don't like you last movie', or 'I loved it!' And it may be only ten seconds - but at least those ten seconds are genuine."

How different all this is from the customary neurotic pleadings of film stars. Perhaps it's his European background; perhaps it's the atmosphere on this film; perhaps it's just him. but Christopher Lambert has a down-to-earth quality you don't often find among successful film actors. Take the subject of interviews. "One think I've learned, "says Lambert, between swigs of beer, "is that when you face a stupid interviewer, you can't be clever. It's like an actor facing a bad actor: he can't be as good as if he were facing Jack Nicholson. And if an actor thinks he can rely purely on his own talent in that situation, he's wrong."

Having achieved his current success after a fairly long apprenticeship ( he was nearly 27 when Hugh Hudson spotted him and used him in Greystoke, Lambert has become used to the stresses of film-making. The film world is glamorous and great fun, but it's also a lonely place: hours of hanging around in one location, then abruptly onto the next, maybe in another country; constantly on call, surrounded by a close-knit circle of technicians, actors, PR people, never making contact with the real world outside; away from home for months at at time... "You have no base, " says Lambert. "You've got no solid ground to rely on. And you go under as a human being. I realize that today even more than I did a few months ago. You need a base, you need somebody. It could be a girlfriend- I'm talking about a serious one- or it could be a place. Because you lose your ground very quickly, that's for sure."

Lambert is unmarried, though rumoured to have recently become seriously attached. "Of course, you can have a girl every night or every week or whenever you want," he says thoughtfully. "But it's not you, it's not you..." his voice trails off. "It's the image they want, that's all. It's not even interesting."

The irony of this is that, as his career accelerates, Lambert has probably never moved around so much. After To Kill A Priest he's off to New York to film a romantic comedy, Why Me? Then it's straight into Highlander 2, filmed probably in London and Arizona-- it hasn't been settled exactly where (another source of insecurity). He has already been away from home for seven months, living in a hotel. He wants to move into his new flat in Paris, he says, but so far hasn't had the time. "But I've got to at least try to go back to a base. And to spend time with people I want to spend time with. Try to have a normal-- a more normal-- existence than just moving around all the time."

I put it to him that he sounds less than completely happy just now, and he nods. "I love life in general with the good side and the bad. But at the moment I would say that I'm a bit depressed. I don't know why, otherwise I wouldn't be. But then you just have to push yourself and wake yourself up. One morning you have to get up and say, 'Listen: cope with being unhappy. Do it.' Sometimes I succeed, sometimes I don't. But I'm trying to live. What else can you do?"

What indeed? Lunch is now over, and we return to the set. Back at the gymnasium the atomosphere is casually bilingual, with shouts of "Silence complet!", "Couper!" and the ubiquitous "Action" echoing around the stone walls. Agnieszka Holland, a diminutive figure dressed in black, stalks across the set chain-smoking and barking directions at the camera crew. In 1981 she left Poland for two weeks; while she was away a state of emergency was introduced and she has never been back.

She and Lambert have formed an especially close relationship. "When I met Lambert," she says of him, "I was quite surprised that he was quite simple and shy. Physically, too, he reminded me of Popieluszko. It wasn't difficult to cast him in this part."

In the real world, meanwhile, life goes on as usual. After filming had begun on To Kill A Priest, the Polish government announced in Warsaw that the real-life murderers of Father Popieluszko have had their sentences reduced for the second time. For many Poles, including Agnieszka Holland, this was depressing news. In the centre of Lille, in a make-believe reproduction of a Warsaw gymnasium, seh was unwrapping another packet of Kools and sizing up camera angles while Christopher Lambert - Father Jerzy Popieluszko - was preparing to be knocked on his backside yet again.