Right in the middle of Koyaanisqatsi, Godfrey Reggio's apocalyptic documentary tone poem – after he has bathed us in torrents of retina-searing imagery, some sped up, some in time-lapse of Earth cannibalising and strip-mining itself– a single, lonely, lost-looking man in a huge crowd stares deeply and directly into the camera lens. It's a shocking moment; a breach of some conventional compact between viewer and subject. But it's also an island of calm amid the chaos.

Faces dotted the other two parts of Reggio's Qatsi trilogy, Powaqqatsi and Nagoyqatsi (the names come from the Hopi language – and the trilogy works around the Native American notion of the world as a once-perfect hoop, now irreparably broken), faces always anchoring a human perspective on the cosmic events.

For his latest work, Visitors, Reggio has done away with almost all background considerations and made a film consisting for the most part of extended slow-motion closeups of people's faces, looking directly at the camera – and at us.

A scene from Reggio's 1982 film Koyaanisqatsi (Life Out Of Balance), part of the Qatsi trilogy. Photograph: Allstar/Cinetext/Allstar

Reggio, who turns 75 this week, spent most of the 50s and 60s as a social activist within the Catholic Christian Brotherhood, working with Chicano street gangs in New Mexico ("like a male nun," he deadpans). "I've always felt that in terms of image," he says, "what is most interesting to us are the faces of other people. A face reveals not only the surface but that which is inside of us. As a young Christian brother, I learned that if I wanted to see my familiar, my ordinary, my normal, for the first time, then I must stare at my face until it becomes strange. That lesson has never left me. The face is the most attractive thing to all of us. So this film not only means to look at the faces, but to have an in-dwelling with the face, so to speak. I wanted a more overt dialogue with the audience, and that can appear confrontational and intimidating: "What am I supposed to think?" That can stimulate a lot of mental activity, which I don't really want, because this movie is a non-mental exercise, directed at the solar plexus rather than the mind. Putting the audience face-to-face with these faces violates all the norms of conventional film-making. It makes us voyeurs at the keyhole."

There's another subject in Visitors, too, of a mournfully abandoned funhouse, a building that Reggio says he shot with the same reverence usually lavished on a movie star such as Tom Cruise, and which he thinks of as "like Pompeii, the ruins of modernity". Perhaps the key image – it opens and closes the film – is of a gorilla named Tisk, into whose soulful eyes we stare for a full two minutes. Only then, after looking deep into the soul of our simian forebear, do we see human faces. Slowly, the usual Reggio oppositions come into play: man in digi-technological ecstasy faces his analogue bestial ancestor, the origins of man face the ruins of mankind, the audience faces the faces on the screen, and Reggio's images face the cyclical, circling music of his regular collaborator Philip Glass on the soundtrack. Thenceforth, as with all of Reggio's movies, one is constructing in one's mind all manner of commonality and opposition, noting repetitions and echoes, the images igniting new ideas as they succeed one another so dizzyingly, while remaining approachable and coherent throughout.

Reggio: when he joined the Catholic Christian Brotherhood, he worked with Chicano street gangs in New Mexico. Photograph: Larry Busacca/Getty Images

Reggio's own face is lean, with yellow patches in his grey goatee, perhaps stained by pipe-smoking. He wears a tight tunic top like a light leather jacket, and looks very much like the socially concerned brother he once was – straight out of The Cross and The Switchblade. I ask how he found his vocation, if that's what it was.

"It's strange to say, but I didn't find it, it found me. I went to a Catholic school, very Catholic family. We model ourselves after the adults we admire, and when I met these men, these brothers, I saw how generous, how joyful, how convivial, how unattached but full of life they were and I thought: gee whiz, maybe I could do that. Little did I know what I was getting into – we all do the right things for the wrong reasons. So at 13 I decided to leave home, at 14 my parents let me, and I never returned. Instead, I returned to the middle ages, and the brotherhood made the Marine Corps look like the Cub Scouts. I became a young zealous monk. My Che Guevara was Pope John XXIII, a pope who never wanted to be pope, and just a guy that moved me to the quick. He said, accept nothing, challenge everything, even the structure of the church. These were marching orders for a young monk, but the institution I was a part of had become deadened."

After a decade of social work and community organising in the barrios of New Mexico, and feeling that his and the brotherhood's aims were out of kilter, Reggio joined the mass post-60s exodus from the church of young Catholic idealists who had hoped for more substantial and meaningful reforms from the second Vatican council.

And suddenly he was a film-maker? "As a brother you don't see movies, because although you're in the world, you're not of it. But another brother showed me a film and said it might change my life, Los Olvidados by Luis Bunuel. Nothing to do with entertainment, but it went right to my soul, I've never had an experience like that. I bought a copy and showed it around and the movie became our church, it had a tremendous impact on the street gang kids who saw it." Alongside Bunuel, he acknowledges one mentor, friend and genius, the Armenian docu-poet Artavazd Peleshyan (The Seasons of the Year): "If fire is the medium, I make sparks, he makes lightning bolts."

In 1972, Reggio helped the American Civil Liberties Union of New Mexico design a media campaign addressing the increasing prevalence of surveillance and privacy issues, and the steady post-Vietnam militarisation of domestic police departments. "It was everything that the Guardian/Snowden/NSA thing is about. We were already talking about it in 1974. This is the first time where kids were being given their universal identifier, their social security number, at birth." He produces a black-and-white newspaper he created for that campaign, picture-heavy on cheap underground press newsprint. On the cover is a photo of a hacked off-looking 10-year-old girl, with a banner of nine numbers across her chest: "That's G Gordon Liddy's social-security number!" Reggio chuckles. "We did a multimedia saturation campaign on it, like regular advertising. We had backers who put up a lot of money and even though we were a radical project we were a solid, heavy presence for a solid month in the media, TV in prime time, radio at drive-time, billboards in high-traffic density areas, insets in major newspapers."

The avant-garde TV spots had almost no words, just images, including one closeup of a human eye that Reggio has called "truly horrifying". And in their violent juxtapositions and clashes of images they contained the germ of Reggio's film-making career: a social conscience crossed with the leanings of a poet and a coruscating Jesuitical rigour. You have to wonder, sui generis as his work so obviously is, where he positions himself as a film-maker?

"I don't, really. I don't come out of academia, either intellectually or cinematically, or the avant-garde. My movies are cine-monads – they don't fit into the rigour of experimental films that you'll see at Anthology Film Archives in New York. I get a lot of grief from those guys, because either they see it as advertising, or it's done too beautifully – there's a whole other point of view on beauty … I don't care where it fits, finally. Let me tell you a little story. In animal mythology, the female lion, her cubs are born stillborn, and she roars them into life, to provoke their hearts with that roar. Beauty and provocation's root words in Greek are closely associated, almost identical: beauty is a provocation and my particular provocation is that I want to show the beauty of the beast."

• Visitors is released in the UK on 4 April