The password for my email has always been "Nashville," because aren't you supposed to choose, for a password, something you'll never forget? I saw Nashville in 1975 at the Pawnee Plaza Theatres, a big event even in the heyday of American moviemaking, heralded by a Newsweek cover and a rare rave review from Pauline Kael, who broke her annual six-month hiatus at The New Yorker to write it. I was thirteen, and although I expected that I might like it, what I saw caused a sea change in my life. It was the first great, grown-up movie that I loved without adult guidance and my passion for it went from the public to the personal: it became a test for friendships, a gauge of my place among my peers, an event by which I marked other events in my life.

Nashville was a movie on the pinnacle of its moment. It caught the complicated mood of a country still reeling from a presidential scandal and defeat in Vietnam, but one that was yet, paradoxically, primed to celebrate its bicentennial, and who would have thought that a movie could capture that? That it did so without resorting to flag-waving or sentimentality remains a kind of cinematic miracle, a happening so spontaneous and alive on the screen that its like will never be reproduced. It's possible that in 1975 we didn't fully understand exactly what we had, but today, I believe, a consensus exists: if the topic is America, then Nashville is the great American film.

Nashville was of course the work of director Robert Altman. His was a singularly fluctuating career: there was the fruitful period that began with M*A*S*H in 1970 and that culminated five years later in Nashville, the long dry spell of the late seventies and the eighties, the one-two punch of 1992's The Player and 1993's Short Cuts, and a final series of missteps before his legacy was brought safely to port with 2001's wonderful Gosford Park and this year's A Prairie Home Companion.

There were other masterpieces besides the five or six mentioned here – McCabe and Mrs. Miller and The Long Goodbye come to mind – and a dozen or more top-notch pictures, but in Nashville we find all the trademark qualities that made Altman an indelible figure in American film: his masterful interweaving of diverse narrative threads, his use of overlapping dialogue, his willingness to grab at the extemporaneous, and the atmosphere of buoyant, cynical – yet strangely hopeful – flippancy that was unique to the director and, with M*A*S*H, brand new to the big screen.

How great a director was Robert Altman? You might almost answer the question by looking at his bombs. In the worst of his pictures – in Beyond Therapy, HealtH, or Brewster McCloud – Altman read like a unfocussed, stoner iconoclast; he could be as self-consciously quirky as a pretentious high school student or as determinedly middle-brow as the makers of Smokey and the Bandit . But not a single film was released under his name that didn't offer something extraordinary, too: a sequence of happy trippiness, a tornado out of nowhere, a wonderful, small performance from Lily Tomlin or Shelley Duvall. More important, you felt Altman reaching for something in almost every one of his films, and what's a failure so long as you're trying?

I mentioned that I expected to like Nashville when I went to see it as an adolescent, but what I hadn't expected was to take my feelings for it so personally. It touched my life, so that when a friend of my mother's said something dismissive about it once in my hearing, I felt as though she were dismissing me. I was likewise surprised at my reaction to the news of Altman's death last week, at the age of 81. Of course Robert Altman had been important to me as a movie lover, but the reaction I felt took place in me personally, the human. Why?

Probably his death taught me what his movie should have when I was thirteen: that, like most of us who go to hear those voices in the dark, the movie-going me and the regular me long ago ceased to exist as two separate things. It's not as though he didn't try to tell me.