"I'm interested in the spirits of people," Ralph Fiennes says. "In the theater, there's the acting part of acting — and I'm not saying that can't be great — and there's the essence." To explore that essence, you need a key, a look, a gesture, an insight that unlocks the person's soul.

"You just want to find that little thing," Fiennes says, narrowing his laser-like sea-blue gaze and the space between his thumb and index finger.

Fiennes — star of films epic ("Schindler's List," "The English Patient"), offbeat ("Spider") and underrated ("Emily Bronte's Wuthering Heights") — has always been good at finding that little thing.

In "The Constant Gardener," Fernando Meirelles' adaptation of the John le Carre novel, which opened Wednesday, the little thing is the manner in which Fiennes' Justin Quayle rises from planting to greet visitors in the lush backyard of his African home, his body bent like a question mark.

Quayle, a middling British diplomat in Kenya, is a seemingly tentative man married to a woman who is anything but. Tessa Quayle (Rachel Weisz) is a fiery relief worker with a penchant for trouble and confrontation, a gardener of souls amid Africa's impoverished, disease-riddled populace.

When she's horrifically murdered in circumstances that suggest she was a less-than-loving wife, it is Justin who must summon the wherewithal to solve not only the puzzle of her death but the mystery of her life.

"But he doesn't become a traditional hero," Meirelles (pronounced Meh-RAY-es) says. "He doesn't become a cowboy. And we'd be disappointed if he sought revenge."

Fiennes agrees: "Justin isn't going to respond angrily ... he's going to carefully say, 'What was she doing?' "

And that makes him a 180-degree turn from the unsettling but charismatic characters that are Fiennes' trademark — the chilling Amon Goeth in "Schindler's List," the Gatsby-esque Charles Van Doren in "Quiz Show," the enigmatic Count Almasy in "The English Patient," the dynamic Mark Antony in a recent London production of "Julius Caesar."

Indeed, Fiennes' Justin is just the kind of playing-against-type role that the Academy of Motion Picture Arts and Sciences tends to reward on Oscar night.

"Ralph deserves an award for this performance," Meirelles says. "He's so relaxed, so natural. We're used to seeing Ralph Fiennes as cold, rational. Here he's lighter. He brings a humanity to the role. He's played a lot of edgy characters. Here he plays an average character."

Fiennes is warm, humorous and blonder than he appears on screen, with a habit of making only brief eye contact and then looking away to answer your question. It is less self-absorption than shyness and a recognition of an actor's extraordinary situation.

"I've had two days now of talking about Justin," he says during an interview in a Manhattan hotel recently. "But you respond to a part. You don't talk about it. You have to be it."

It helps to be as precise as possible with "The Constant Gardener," because the movie is intricately layered. It's a thriller in which no one is quite what he seems.

It's a complex medical drama that offers a devastating indictment of pharmaceutical companies and governments testing unproven drugs on developing nations.

It's a political movie, with topical references to the war in Iraq and the conflict in Darfur in Sudan that has ground women and children into the African dust. (In updating le Carre's novel, which was published in 2001, Meirelles says he and screenwriter Jeffrey Caine were helped by the author himself, who had final approval over every aspect of the film and suggested many lines.)

"The Constant Gardener" is just one of a half-dozen movies in which fans will see Fiennes this year and the next. He lends his voice to the big-haired, gun-toting Victor Quartermaine in the stop-motion animated feature "Wallace & Gromit: The Curse of the Were-Rabbit" (Oct. 7) and plays a very different diplomat, blind and embittered in '30s China, in Merchant Ivory's "The White Countess" (Nov. 11).

But the most-anticipated film is "Harry Potter and the Goblet of Fire" (Nov. 18), in which Fiennes embodies the serpentine Lord Voldemort, arch enemy of hero Potter and "the most evil of evil of evil of evil of men," the actor says, shaking his head side to side before slipping on the spine-tingling silk of Voldemort's voice.

He's not a big Harry Potter fan. But then, flippancy gives way to the hallmark Fiennes seriousness.

"I'm not a psychiatrist," he says. "But I do think childhood is the answer. And thank God, J.K. Rowling has done the work for us. Lord Voldemort is a rejected child. Once I understood that, I could locate the rage."

It's that little thing, you see. And another essence is unleashed.