Along time ago, Harvey Keitel was afraid of the dark.

And then he was a 17-year-old Marine, terror-stricken in a night combat course. It was so dark out, Keitel says, that he couldn't see his hand in front of his face. The instructor was a veteran of the Korean War, and he made a promise to his uneasy recruits. "He said, I'm going to teach you to live in the darkness. You're all afraid of the darkness. We're all afraid of what we don't know, but I'm gonna teach you to know the darkness, so that you're no longer afraid of it.'

"He could tell us those words because he had experienced the darkness. He had experienced that terror in a war," Keitel says. "In the years that came, that is one of the essences of all the mythology and philosophy that I have read."

It is also one of the essences of Keitel's work; the actor has made a career out of plunging into darkness. Keitel does not merely play bad guys. Villains are too easy, too obvious. "I'm drawn," he says, "to stories that concern that difficult journey to do what's right."

In Spike Lee's "Clockers," which opens today and is based on Richard Price's best-selling book, Keitel plays Rocco Klein, a homicide detective determined to pin -- at any cost -- a drug dealer's murder on the brother of the man who confessed. The character, Keitel says, suffers from "the disease of self-righteousness." In Jane Campion's "The Piano," Keitel portrayed an unrefined New Zealand settler who realizes he has turned the woman he loves into a whore. And in Quentin Tarantino's debut, "Reservoir Dogs," his Mr. White was honor among thieves; "the guy was a killer," Keitel explains, "and he had the need to be a hero to somebody."

"There's a common theme that unites the stories," Keitel says. By way of explanation, he quotes from the opening chapter of "David Copperfield": "Whether I shall turn out to be the hero of my own life, or whether that station will be held by anybody else, these pages must show."

Such challenging roles come after years in which Keitel seemed doomed to play bit-part bad guys. Again and again, his roles were patterned after the Brooklyn toughs he created with director Martin Scorsese in the films that made both their reputations. During the '70s, Keitel didn't enjoy the adulation heaped upon his "Mean Streets" and "Taxi Driver" co-star Robert De Niro. Instead, he remained a footnote, an actor's actor who contributed remarkable performances to such less-than-famous films as "The Duellists" and "Fingers."

But the past few years have been different for Harvey Keitel. At a time when many of his peers seem to be resting on their laurels, he has turned out some of the best roles of his career. Now 56, he's doing more interesting work than any other actor of his generation -- who else could you imagine playing the depraved, drug-addicted cop of Abel Ferrara's "Bad Lieutenant?"

Intensity is part of his stock in trade, and even his recent supporting roles have been indelible. His corpse-dispensing "cleaner" in 1993's "Point of No Return" was so matter-of-factly menacing that Tarantino cast him as another "cleaner" in "Pulp Fiction." His riveting portrayal of mobster Mickey Cohen in Barry Levinson's 1991 "Bugsy" garnered him an Oscar nomination for Best Supporting Actor. But Keitel insists that he was neither thrilled with the nomination ("I was so happy," he says sarcastically, "I went running down the street singing Zip-a-Dee-Doo-Dah' ") nor disappointed that he lost out to Jack Palance in "City Slickers."

"The only true recognition is in the work itself," he says. "You don't need to be nominated for an Oscar to be recognized for your work."

No wonder Keitel has become an icon of the Quentin Tarantino crowd. Off screen, Keitel's support for "Reservoir Dogs" -- he ended up co-producing -- lent Tarantino legitimacy and financing. On screen, Keitel provided a certain gravity and depth.

Keitel currently can be seen in Wayne Wang's "Smoke," playing the affable owner of a Brooklyn tobacco shop; he was also the impetus behind "Blue in the Face," a collaboration between Wang and "Smoke" screenwriter Paul Auster that opens here this fall.

Tarantino. Wang. Lee. Ferrara. Campion. He's also in Robert Rodriguez's next film, "From Dusk Till Dawn." Keitel is sought after by some of contemporary film's most provocative directors. As for those bad years -- well, they're over. Keitel once said that the shortage of substantial parts that came his way was "humiliating." Now he sees things differently: "My early career was exactly what it should have been. I was ready to do certain things; I was not ready to enter into other places in my own soul that would have given me the materials that I needed in order to make my work deeper. I wasn't ready. It took time. It took experience. It took struggle, a great deal of struggle, to find out about who I am.

"I might say that in my life I'm at maybe the most interesting time of it. In my career, obviously I have attained more control over its destiny," he says. "We make our destiny." Young and Restless

"Smoke" and "Blue in the Face" take Keitel back to Brooklyn, where he was born and raised by Eastern European Jewish immigrants. They lived in Brighton Beach, where his father worked as a hatmaker. His parents also owned various luncheonettes over the years; summers, they rented an Atlantic Beach concession stand, where they sold hot dogs, knishes and sodas.

When he was 7, Keitel developed a stutter that would plague him for years. "It was a huge, huge, deep, deep embarrassment, the object of humiliation by other children," he says. "It took years to go away. I still stutter at times." It doesn't bother him anymore, though. He has come to accept that the stutter was "something that occurs as the result of something else. . . . It's sort of a road to your identity, it's a clue about something, it's a clue about disturbance."

By the time of his bar mitzvah, Keitel was already something of a tough: The rabbi escorted one of Harvey's friends out of the synagogue because the boy was wearing peg pants and a checkered cabana-style jacket. Keitel skipped school to play pool -- "I was so-so" -- and hang out in front of sweet shops and diners. Eventually he was expelled for truancy. "I was a great student," he says. "I failed everything, but I thought I was a great student.

"I didn't like anything, except hanging out with my friends. I didn't want to read. I didn't want to study. I didn't have the concentration, I didn't have the focus. I was just upset. And being upset doesn't afford one the patience necessary to learn anything."

What was he so upset about? "I'm not going to discuss those things," he says coldly.

There is a long pause.

"I was upset with those familiar things that perhaps any young person . . . would identify."

Keitel joined the Marines, where he began to learn about the darkness. "I learned things there that were the beginning . . . of a spiritual journey."

After returning to New York, he sold shoes, then became a court stenographer. When he was 23, at the suggestion of a co-worker, he started taking acting lessons. Over the years, he studied under Stella Adler, Frank Corsaro and Lee Strasberg; when responding to questions about his craft, he likes to cite director Richard Boleslavski's classic text "Acting: The First Six Lessons."

What has he learned from all his acting classes? "A technique to enable the actor to enter into the unconscious, the subconscious -- basically, that," he explains. "A technique to re-create experience that you're aware of and experiences you are not aware of."

There's some plain old pretending in there, too. As the tobacco store owner in "Smoke" and "Blue in the Face," he plays the kind of chain-smoker who grimaces slightly each time he lights up. But Keitel wasn't exactly smoking. He is a reformed smoker and was afraid he'd get hooked again. So he puffed on the faux English cigarettes known as "vegetable sticks" -- which are difficult to draw on, he says, and smell like burning lettuce.

His first film was also Scorsese's first, "Who's That Knocking at My Door?" (1968). The two collaborated on three other films during the '70s: "Mean Streets," "Alice Doesn't Live Here Anymore" and "Taxi Driver." Since then, Keitel has worked only once for Scorsese, in "The Last Temptation of Christ." Among film buffs, Keitel's most famous misstep is a role he chose not to take. Francis Ford Coppola had cast him as Captain Willard in "Apocalypse Now" -- the part that would eventually go to Martin Sheen. But Keitel balked at the contract; he was concerned that Coppola, who was trying to accommodate Marlon Brando's schedule, would not free him in time to make his next film. Keitel refused to sign, and Coppola fired him. And after all that, the other movie never got made.

Keitel says if he could turn back the clock, he would have made his own films early in his career. "Acted, wrote, directed. I had some enormously talented men and women around me when I was a young actor," he says. "That's something I can give to the young actors, writers and directors: Make your own films. Don't wait for Hollywood. You don't need Hollywood to make your own films."

During the filming of "Smoke," Keitel suggested that he and other characters work on improvisations off-camera. Wang was so impressed with them that he and Auster wrote "situations" for the actors to improvise; "Blue in the Face" consists of these scenes as well as additional footage shot later.

In both films, Keitel plays the odd but benevolent Auggie Wren, who shoots a single photograph of his tobacco shop's street corner every morning at 8. Keitel describes Auggie as a "philosophical storyteller." Auggie, he says, "understands something about living that is quite unique. Which is that, on his own corner, in his own neighborhood, perhaps all one needs to know will take place. And part of me believes that.

"I'm speaking metaphorically when I say on your corner,' " he adds. "You might want to walk around the block." The Metamorphosis

Like most of us, Keitel is plagued by insecurities. What is he insecure about?

"I will escape from that question by invoking van Gogh," he says, pronouncing it "van gog." "He wrote in letters that when he regarded the canvas, it always looked at him and mocked him and said, You can't paint. You have no talent. You are not an artist.' "

But he is a fearless actor. Full-frontal male nudity is rare in American cinema, but Keitel stripped down for both "The Piano" and "Bad Lieutenant."

"I never do naked scenes," Keitel says stubbornly. "An actor does not do nude scenes. I told a story, and it's my obligation to tell that story as honestly and as deeply as my ability will allow me." Pause.

"I'm not going to discuss nudity in films," he says. "People should not be nude in films. People should tell stories that mean something to them."

It is difficult for Keitel to explain how he prepares for a role. "I can tell you in a very general way, because the technique remains the same and you apply that to any role you play," he says. "You analyze the text, you try to understand the social situation of the story, what that means, what was the morality of the time, the religion and so on."

For his Judas Iscariot in Scorsese's "The Last Temptation of Christ" -- the role that jump-started his stalled career -- Keitel read Nikos Kazantzakis's novel as well as the writings of Christian historian Elaine Pagels. (He was so enamored of Pagels's work that several years ago he insisted she be present at an interview for Vogue magazine. "As a result of that," Keitel says now, "women who were unfamiliar with Elaine Pagels became familiar with her work.")

To prepare for "Clockers," he hung out with Jersey City homicide Det. Larry Mullane, the real-life inspiration for author Richard Price.

Keitel says he was drawn to "Clockers" because it reminds us of something we need to hear over and over again: "That the result of poverty and hopelessness is always going to be violence."

Ah, violence.

"Reservoir Dogs" is an exceptionally bloody film; most of it takes place in an empty warehouse in the hours following a botched diamond heist. As an undercover cop lies bleeding to death in a corner, another policeman is tormented by one of the gang. The excruciating torture scene culminates as the gangster cuts off the cop's ear -- while dancing to an old pop song, "Stuck in the Middle With You."

"Violence should always be portrayed painfully," says Keitel. "A lot of people responded to the pain of the violence in Reservoir Dogs.' It was vivid; that's the way it should be. It should be disgusting, the way that real violence is."

Keitel swears he's on the side of Washington politicians in their war against screen violence. "Bob Dole is right to raise the question. I'm glad he raised it. We are always raising it ourselves," he says. "We were always very aware of our responsibility not to cross the line into gratuitous violence -- no one wanted to do that.

"If anything is gratuitous, it's that people portray violence so that it is acceptable," he adds. "The Tarantinos, the Scorseses, who are trying to portray the experiences of humanity honestly -- we must give these people our support. Even if we throw up watching it."

There is a scene early in "Reservoir Dogs" in which Mr. Orange (played by Tim Roth) lies bleeding on the warehouse floor, squealing and moaning in pain as Mr. White (Keitel) stands helplessly beside him.

Mr. Orange: I'm so {expletive} scared. Can you please hold me?

Mr. White: Yeah, sure.

Almost imperceptibly, White's eyebrows lift. But he lies down next to the boy and holds him anyway. A few minutes later, he awkwardly combs the kid's hair.

Keitel says he once heard an interview with Hitler's favorite architect, Albert Speer, who said that the most frightening mistake humanity could make would be to think that Hitler was crazy: "You get my point?" he asks. "To think that every criminal has no love in him, no need to be a hero, no need to do what's right, would be a tragic mistake to make."

Several years ago Keitel spoke at a Holocaust commemoration in Hamburg, Germany, where he unexpectedly found himself invoking the 1970 shootings at Kent State University. "I would have fired had I been ordered to fire," he says. "As a young Marine, I would have fired upon those students myself. So make no mistake that we' are them.' And to separate us and them would be a mistake, because that would not teach us the way to approach ourselves, and therefore, them." Secret of the Dark

As Abel Ferrara's "Bad Lieutenant," opens, the title character is driving his two young sons to school, and he's fuming because they're late. "Daaaad, it's not our fawlt," they protest, and their words rush together as they explain Aunt Wendy told them to take out the garbage and then she hogged the bathroom all morning. The lieutenant directs a blast of Keitel fury at his boys: "When it's your turn to use da bathroom, you tell Aunt Wendy to get da {expletive} outta da bathroom. Whaddayou, men or mice?"

Later in the NC-17-rated film, when the lieutenant is bad he is rotten: In between placing bets with his bookie, he smokes crack, snorts coke, shoots heroin, beds junkies and forces a teenage girl caught driving without a license to mime oral sex for him as he masturbates. Eventually, he undergoes a redemption of sorts, one brought about by a nun's forgiveness of the two boys who savagely raped her.

Keitel says that role was a result of his descent into the darkness. "What that Marine was teaching -- it's not that you are not scared in the nighttime, it's that you learn about your fear and the darkness. That fear becomes different, and you can work with it."

" Bad Lieutenant' came at a certain point on this journey," he says. "It seemed to me like a destiny to arrive at that place." CAPTION: The tough acts of Harvey Keitel, clockwise from top left: with Samuel L. Jackson and John Travolta in "Pulp Fiction"; in "Reservoir Dogs"; with Holly Hunter in "The Piano"; in "The Duellists"; and in "Bugsy." CAPTION: Beyond the bad guy: Harvey Keitel in "Bad Lieutenant."