The star of Flight, whose career has gone from Malcolm X to action hero, talks about why he shuns the showbiz spotlight and reveals that a prophecy in a beauty salon first led him to take up acting

Denzel Washington rolls into London on a chill January morning and the snow-storm that follows can barely compete. He has a cup of coffee at his elbow and a frosted window at his back. He's talking up a blizzard, he's talking to keep warm; spouting off in great, rousing, charming gusts. "You're not even having to ask me any questions," he marvels happily. "I'm just talking." Afterwards it will strike me that this is exactly how he likes it.

If one judges an actor by how adeptly they immerse themselves in the roles that they play, then Washington is the industry's sasquatch; the performer who would not be caught. Over the past quarter-century I have known him as Malcolm X and Easy Rawlins, as a runaway slave and a corrupt LA cop. I've seen him gassing with Oprah, and collecting his Oscars, and yet could probably walk past him in the street without so much as a backward glance. Washington, for all his verbal flurries and on-screen explosions, is a Hollywood actor who hides in plain sight.

In person he is trim, athletic and casually self-assured, a 58-year-old man who could pass for 40 and a far cry from the human wreckage he plays in Flight. Robert Zemeckis's film casts Washington as William "Whip" Whitaker, an alcoholic, coke-snorting airline pilot who performs a daredevil rescue and is then hauled before the jury to account for his actions. The actor explains that he researched the role by working on flight simulators and sitting with pilots. The younger trainer felt the plot was unrealistic and potentially damaging to his airline. "Ah," he said. "We never have this kind of problem. Drunken pilot, no such thing." The senior trainer told a different story.

Small wonder Flight has brought the star an Oscar nomination. The film succeeds as both a big, brassy Hollywood entertainment and a knotty, perfectly played study of a man in crisis. It also possesses a bracingly ambivalent attitude to drug abuse. Flight, it seems to me, contains not one but two acts of extraordinary courage and clear-sightedness. And yet on both occasions Whip is loaded to the gills; in the grip of an addiction that seems, in each instance, at least, to be as much angel as demon.

Washington frowns. He's not sure he quite views it that way although yes, he can understand why some may feel differently and yes, he can see the appeal of such stimulants; a little boost to achieve the right altitude. He thinks it's probably a hazard of any job – be it aviation or acting – that involves long stretches away from home and brief periods of intense activity. But on balance it is not for him.

"I did a movie in London in the 80s, For Queen and Country, and there was a scene where we had to drink," he recalls. "So me and this other young actor said: 'Man, let's really drink.' We had some scotch and thought we were playing it pretty cool. And the director said: 'What's wrong with you guys?' We thought we were wild but we were really just dull. The director was pissed off and it taught me a lesson. That was the last time I ever drank while working. Because it doesn't work. I've worked with actors who've crossed that line and it's just not worth it. You think you're giving 100%, but all the while your level is dropping."

From the sounds of it, Washington got his own wild years out of the way early. In his teens he ran with a rough crowd and flirted with disaster. He had no idea what to do with his life, so he enrolled on a pre-med course, then transferred to political science and then to journalism. But his grades went south and the college kicked him out.

"Acting was my calling," he says today. "The year I started acting there was a woman in my mother's beauty shop, a kind of seer if you like, who kept looking at me in the mirror. Finally she got a piece of paper and wrote 'prophecy' at the top. She said: 'You are going to travel the world and preach to millions of people.' Now, bear in mind that I'd just been kicked out of school. I said: 'You see anything there about me being let back into school?'" He chuckles. "That was March 75 and September 75 I started acting. I still got that piece of paper too."

Denzel Washington with Tom Hanks in Philadelphia. Photograph: Allstar/Cinetext/Tristar

He bagged his first role in a musical down at New York's Lincoln Center which was terrible ("because I can't sing") and his second playing the lead in Eugene O'Neill's The Emperor Jones. By his early 20s he had an acting agent and a role in a TV movie called Wilma, where he met his wife, Pauletta Pearson. "The rest is history," he laughs. Three decades on, he is still married to Pauletta.

If there is a bedrock to Washington's life and career it may well have been provided by his father, the Reverend Denzel Hayes Washington, who juggled day-jobs at a department store and the water board with Sunday services as a Pentecostal preacher. The actor was raised in a religious household, strayed briefly in his youth and is now firmly back on side. In 1995 he donated $2.5m to help build a Pentecostal cathedral near his Los Angeles home. Regular attendees include Magic Johnson, Stevie Wonder and Washington himself.

Sometimes he wonders if there is still more he can do. "I remember some years ago asking my pastor: 'Do you think I'm supposed to be a preacher?' And he said: 'Well, you are. You have a pulpit of your own.'" Washington gulps at his coffee. "That's not to say that I'm preaching, necessarily. I don't want to tell you what you need to do. I mean, I'm not turning it up to 10 when it comes to being correct, I'm not that guy, I like my wine."

He does, however, have a firm moral stance on the roles that he takes. Washington rejigged his Oscar-winning role as damned, dastardly Alonzo Harris in Training Day so that the character's come-uppance was more severe and admits that he tried to do the same with Whip in Flight. The film, he feels, lets Whip off too lightly.

Denzel Washington with Chris Pine in Tony Scott's Unstoppable. Photograph: 20th C Fox/Everett/Rex Features

I tell him that Flight actually struck me as a deeply religious movie. Near the end, for instance, the hero is even shown calling on God for help. "That's right," says Washington. "But by that point he's got nowhere to hide. It's like what's going on with this Lance Armstrong stuff now. The jig is up. And that line 'God help me' was very important to me. As an arc as much as anything. You see this butt-naked, coke-sniffing, weed-smoking guy finally get to the point where he can say: 'God help me.' He gets his life back."

I suggest that an upright, Christian movie star must be something of a rarity in the modern-day Gomorrah of Beverly Hills and he shrugs and frowns and insists that I'm confusing the town of Los Angeles with the industry itself. "But actually, even within the industry, I don't have any actor friends," he concedes. "My friends are old friends. One's an ex-music guy, the other's a restaurant owner and the other's an ex-pro ballplayer."

Why hasn't he made any actor friends? Washington chortles. "Because I don't make friends! Maybe I'm not a butt-kisser, maybe I'm not a schmoozer. I'm not about to go to a party to try to get a job. And then when you have children, the other friends become other parents. We'd coach baseball or basketball. My wife and I were raised right. I don't want movie-star friends. And," he says, thinking it over, "being African American, there were no big movie stars to hang out with anyway, not when I was starting out, they were just the third guy from the back! For whatever reason, I never befriended any white actors."

It is perhaps a measure of Washington's success that he has now reached the point where race is just another part of his actor's toolkit. Back in the early days he gained an Oscar nomination for playing the activist Steve Biko in Cry Freedom and won the best supporting actor honour as an escaped slave in Glory. But in many of his subsequent pictures (including the five blockbusters he made with the late Tony Scott) the issue of race has been allowed to remain in the background. Whip in Flight is defined by many things: by his alcoholism and his heroism and his fraught relationship with his teenage son. The fact that he is black seems entirely incidental.

Denzel Washington playing Malcolm X in Spike Lee's biopic. Photograph: Allstar/Cinetext/Warner Bros

Our time is almost up; the publicist hovers anxiously at the door. And still Washington won't stop talking. Acting was a salvation of sorts, he explains, in that it was the first thing he tried that he was able to do. "I liked the world and I was successful at it."

Who knows why his career turned out as well as it did? Fate, luck, talent, or some combination of the three. "I had one six-month period in 1982 when I couldn't get a job. I had done a movie called Carbon Copy and then I did a play and then I had these six months where nothing happened, where I started looking at the department of recreation and thinking: 'Acting's not for me.' And then I caught a play about Malcolm X, off-off-Broadway in New York and I haven't been unemployed ever since."

By this point the publicist is practically dragging me from the room. Washington gets up from his seat and walks me there himself. "I'm a working actor!" he hollers, as if by way of introduction, as though that's all I need to know. "What's a celebrity anyway? Paris Hilton's a celebrity. I'm just a working actor." He shakes my hand and slaps my shoulder. Then the door swings shut and it hides him from view.

• Flight is out in the UK on 1 February