Oh, no, look out. There she goes again.

Tessa is about to let 'er rip. It's so embarrassing. Doesn't the poor girl know that one just doesn't do certain things? Not Tessa. She has no sense of what's appropriate.

So Tessa arises and begins to assail the poor chap from the Foreign Office in the middle of his dreary lecture. Doesn't he realize, she demands, that British foreign policy is a sham driven by corporate greed and narrow-minded nationalism? Doesn't he realize that people are dying, dying, and all the government cares about is keeping football hooligans employed in Scottish factories? Doesn't he see the deep, the profound, the complete immorality of it all . . .

ZZZZZZZZ-zzzzzz-ZZZZZ!

Tessa (played brilliantly by Rachel Weisz) breaks down in tears at her audience's indifference and her quarry's affronted bafflement. The scene is from Fernando Meirelles's version of the John le Carre novel "The Constant Gardener," which opened this week; it's a fascinating grown-up film that examines, among other things, the nature of zealotry.

He's not a familiar figure like a cowboy, a samurai or a naked lady. He's not even up there with detectives, pilots, presidents and lawyers. But still: Movies quite often have a sniff of politics to them, and a figure they seem to come back to time and again is the moral crusader, the fellow who dreams of things as they can be in their idealized state. Noble as that sounds, the truth is, such men and women have been, over time, rarely portrayed as noble.

"The Constant Gardener" is a case in point. In fact, if the film had left off where I stopped describing, you could say it represents a clear theory of zealotry. It begins with a close observation of the phenomenon. The zealot -- strident Tessa, hurling thunderbolts of rage at her queasy target -- isn't just a lass who cares; rather she cares perhaps too much. She can't leave it alone. Drama is etched in her forehead, her eyes track injustice everywhere, her medium is anger, her weapons scorn and sarcasm, she knows how right she is, and her signal frustration is that nobody cares. They are sooooo-oooo stupid!

The zealot, political or otherwise but usually political, is usually a very unhappy camper. Breakdown is never far away. The voice cracks, the eyes well with tears, the sentences, so propelled by anger, lapse toward incoherence. When the inevitable rejection is encountered, the whole body seems to deflate, and every line seems altered. It's the weight of carrying all that misery around, finally too much to bear. Weisz gets this brilliantly, and when her Tessa deconstructs to a moody puddle after the hoots of the crowd wear her down, you can feel that her pain is soul-deep.

We know: She's got some screws loose. Though implicit, the psychological reasons are transparent. We can assume it all goes back to the family. If our boy or our girl has some major issue with a cruel and tyrannical dad or mum, inevitably that fury transfers itself to the state. Thus your zealot is always someone working out his surrogate dramas in the arena of politics as pure compensation. Had the poor kid been hugged once or twice, told a bedtime story or two, tucked in, allowed to cuddle on stormy nights, none of this would have happened, from Tessa's hissy fit at the man from the Foreign Office to the storming of the Winter Palace, the execution of the Kulaks and the show trials. It all belongs on the same continuum and once there, it's really only a matter of degree.

Other messinesses attend this personality disorder, and to some degree the first part of "The Constant Gardener" is a clinic on the condition. It seems always a part of the package that such angry people are usually somewhat messy in their personal life. Tessa is no exception; she's almost a stereotype -- she's promiscuous, flamboyantly so, and her eventual husband, that dreary Foreign Office chap Justin, played by Ralph Fiennes -- goes into the books as a cuckold, the chump who's last to know. Then, too, she's reckless: She'll beard anyone, anywhere, anytime, and begin to argue. She'll venture upcountry and unarmed in pursuit of something she perceives as "truth," and it's only a matter of time before, sooner or later, she gets knocked off. She's a mess.

This is actually a fairly common portrait in the movies. Think of the many other radicals the flicks have portrayed as loose cannons with messy lives, full of small treacheries against people while yapping moral triumphalism in favor of some mythical animal called The People.

In "Silkwood," Karen Silkwood was a mess; in "Norma Rae," Norma Rae was a mess. Think of the angry young man, Perchik, the one the second daughter goes off to marry in Siberia in "Fiddler on the Roof." Really, didn't you know he was a mess and the poor girl was heading off 2,000 miles to the Arctic Circle to her own doom? You knew that, didn't you?

Or think of the Big Daddy of all radicals in radical movies, Warren Beatty as John Reed in "Reds." The movie aimed to be a love story, a tender tale of kindred spirits finding themselves against a backdrop of political tumult, and yet, even with the benevolent Beatty himself behind the camera, it seemed more an indictment of foolish, beautiful, romantic sensualists hellbent on self-indulgence and the pleasures of flesh and fame rather than the rigors of political or revolutionary discipline.

Then there's "Frida," the Salma Hayek bio of the radical Mexican painter and feminist. Did this woman have troubles or what? Again, both star Hayek and director Julie Taymor meant to be kind to the artist, whose vivid style and association with her lover Diego Rivera lit up left-wing circles in the '30s. But try as they could, she seemed like a twisted sister off on a self-dramatization crusade, who used a bus accident as a tool for constant manipulation of those around her, turning many things into one-act plays starring her own self. Some would come away from that film thinking: She's not a part of the solution; maybe she is the problem.

The same is true of Nicole Kidman in the recent "The Interpreter." In this film, we begin with an image of a completely rational young woman who seems professional, astute, in control; but it turns out that her psychology has been created by trauma in a fictional African country much like Zimbabwe and that the more radical she is seen to be, the more mentally disturbed and distraught she is. Finally, she's revealed as a secret assassin, or so it seems, and the movie's dramatic fulcrum turns on her willingness to commit an act of terror. But she doesn't. She withdraws and, symbolically, bows to the wisdom of the United Nations (where the movie has been set). Her decision not to shoot was seen almost as a "recovery," as the final step of a therapeutic 12-step program.

The zealot torch is getting passed to Charlize Theron next; we don't know just what flavor of radical she'll bring -- the movie, "North Country," opens next month -- but a peek at the Norma Rae-esque photos of her character standing tall in The Big Meeting, as a miner who wins the first major sexual harassment case in the United States, gives an idea.

In fact, I can think of only one reformer who was without psychological back story, nervous disposition, teary eyes, stammering voice: That was Clint Eastwood as an avenging, gun-toting environmentalist in 1985's "Pale Rider." He didn't have time to stammer or cry; he was too busy shooting folks.

Frequently, in fact, the figure of the zealot is used to contrast with a more admirable hero. That was very much the case in Ron Howard's recent "Cinderella Man," set in the depths of the Great Depression. Russell Crowe played the middle-of-the-road Jim Braddock, all-around good guy. But in one of his interruptions in his fight career, when he's working on the docks, he meets Mike Wilson, played by the Irish actor Paddy Considine, a true rad, almost a Wobbly, maybe a Red. He's a dockworker enraged at the breakdown of capitalism and the usurpation of the cops as Cossacks for industry, while he and his starve, beg for handouts or scrabble for the few jobs available.

Hollywood to radicals: Drop dead.

For his effort, Mike gets squished by a police horse during a raid on the Hoovervilles in Central Park, and Jim learns a lesson: Go the middle road, don't let your anger destroy you, play by the rules, ride the system, win. He does.

There wouldn't have been a movie elsewise.

One can see from all this why the radical is a recurring figure in the canon. Like the cowboy, the samurai and the naked lady, they're almost inherently dramatic. They're always found at the spear point of conflict. Their passion usually makes them charismatic and big stars like to play them, because they move the plot, star in the action sequences, kiss the girl and, afterward, cash the check and accept the award.

But they also get on your nerves fast. All that moral certainty, all that unflinching rectitude, all that stubbornness. Some of them really make you sick. Come on, Brubaker, when Robert Redford played him in the prison reform movie of the same name: what a stiff. Erin Brockovich? With that cheap-shot smugness and improbable beauty? Pain in the butt. In fact, if you look hard, two of the biggest romantic crusader-losers in movie history (where it intersects with literary history) were found in earlier films, both courtesy of a gentleman who regarded do-gooders as dangerous: Joseph Cotten in 1949's "The Third Man" and Audie Murphy in 1958's "The Quiet American." The fellow who concocted them both and detested them both was Graham Greene, the great British writer who found do-gooders sublimely irritating.

The funny thing about "The Constant Gardener" is that at a certain point it makes a Page 1 retraction and sends itself to rewrite.

It turns out the initial vision of Tessa was a ruse, meant to cleverly stoke popular preconceptions and thus acquire power in the reversal. The movie tracks a process of education and the arrival of knowledge. By movie's end, its portrait of Tessa the Radical has been wholly rearranged.

The progression of the plot has been husband Justin's journey into the heart of his own wife, where he makes two astonishing discoveries. (SPOILER ALERT!) The first is personal -- it's that indeed, she loved him and her "promiscuity" was a delusion of others. In fact, she loved him so much that an alternative theory soon establishes itself -- that she led a secret life to protect him and his career from the taint that she knew her discoveries would entail.

And that of course leads to the second revelation: That far from being a screwball angry at a vanished or indifferent parent taking her for granted, she has rational politics. They're cause-based, not anger-based. She's examined the policy, she's understood the science, she's read the documents, she sees the implications.

This is, by the way, what might be called a counter-theory of the zealot, which is why "Constant Gardener" is such an ideal study tool for the phenomenon: It encompasses both the theory and the counter theory, ultimately endorsing the latter. It holds that the radical is not a crazy hater who needs to tear down out of narcissism, to avenge her own slights. She takes no pleasure in destruction; it's that she takes no pleasure in injustice, and if she sees injustice everywhere, it's because injustice is everywhere.

A loose cannon with a messy life: Rachel Weisz as a woman who won't ignore injustice in "The Constant Gardener." Right, Clint Eastwood as a different sort of reformer in 1985's "Pale Rider."Always at the spear point of conflict: Sally Field makes a stand in the pro-union "Norma Rae," from 1979. Below, Charlize Theron as a courageous and outspoken mining union member in the upcoming "North Country."Coming across as a twisted sister on a self-dramatization crusade: Salma Hayek in 2002's "Frida."