A few weeks after I met Roy Lichtenstein in 1997, he died of complications arising from pneumonia. His going was unexpected and shocking. Only 73, Lichtenstein had been in excellent health and enjoyed an eventful year alive with new possibilities. That summer, he had shown a roomful of clangorous, brilliantly coloured sculpture at the Venice Biennale. He had also had a solo exhibition in Boston of paintings inspired by Oriental art, which suggested a new direction. His work was defiantly on the move.
As was Lichtenstein himself. When I arrived at his New York studio, a window-barred building at 745 Washington Street to the west of Greenwich Village, I was confronted by a scowling guard dog with the word "GRRR!" erupting in bold capitals beneath him. It was, of course, a comic-book animal, endearing rather than ferocious. But the energy embodied in the dog was shared by the artist who created him. While I was there, Lichtenstein didn't sit down once. The so-called Grand Old Man of American Pop Art turned out to be lean, understated, soft-spoken and spry. His white hair tied back in a neat pigtail, he paced round his wide, barn-like studio in T-shirt, jeans and trainers. When I photographed him in front of an enormous new painting, part of a serene series called Landscapes in the Chinese Style, he paused only for an instant. He still possessed the formidable energy that, a generation earlier, had enabled him to make such a controversial and crucial contribution to the Pop Art movement.
The other paintings ranged on easels around the studio suggested that he had no intention of slowing down. Most were interiors, where young women ruminated among plants, furniture and bookshelves or lingered mysteriously at the side, sliced off by the canvas edge. I took a photograph dominated by a painting called Interior with Nude Leaving, which will be included in Tate Modern's forthcoming retrospective. The elusive woman on the left of Interior with Nude Leaving is clearly related to the flaxen-haired beauties who thronged his early Pop pictures in the 1960s. The dots, diagonal stripes and thick contours deployed in this new painting are consistent with the style he had developed 36 years before (also to be represented in the Tate's show). But the colours are lighter, cooler, more airy. There are no speech-bubbles floating in space, and the exiting woman can barely be glimpsed.
Near the centre of his immaculate studio, a former ironworks converted eight years earlier, Lichtenstein's sofa was strewn with evidence of his interest in visual culture at its highest and lowest. My gaze fell on big books about Cézanne and Matisse. But I also spotted a copy of Falling In Love magazine, subtitled Yesterday's Sweetheart. It is difficult, now, to imagine just how heretical such source material would have seemed in 1961. That was the year Lichtenstein's work underwent a radical change. Suddenly, he rejected the abstract art he had been making, and began deploying comic-book imagery.
At the beginning, he was as startled as anyone. Recalling the decisive moment, he told me: "I felt my change from the 1950s work was a rupture, a huge shock – like the one Picasso delivered in 1906. For me, it started with the idea of painting clichés, jotting down little drawings of Mickey Mouse and Donald Duck. The cliché gave my work a certain power. It was brave, risky and so far from anything I'd been taught in art schools. It was saying something about real life, and it wasn't done as a joke. But I knew that it couldn't be taken seriously."
Last year, one of these Pop paintings, Sleeping Girl, sold for a record auction price of £28.2m. But in the early 1960s, Lichtenstein's heretical new art was anathema to a generation reared on Jackson Pollock, with his heroic emphasis on a wholehearted, muscular interaction between canvas and paint-splattered artist. Lichtenstein's reliance on cheap commercial images seemed an appalling rejection of everything that had made post-war American art so admired around the Western world. In 1964, following his notorious show at the Leo Castelli Gallery in New York, Life magazine published a feature headlined: "Is he the worst artist in America?"
Lichtenstein found himself shunned. "The US museums weren't looking at my paintings at all – they hated them, irredeemably. People metaphorically threw up when they saw my work! They thought I was enlarging comics, or just copying them." The Tate recognised his talent very early, and was wise enough to purchase his immense 1963 canvas, Whaam!. Its impact is still overwhelming. This Boy's Own adventure strip, of one fighter plane attacking another, has been blown up so much that it hits us in the face. With its big screening dots and diagonal stripes, the image assaults us, its apocalyptic dynamism a comment on the way young men get caught up in war as a deadly game.
On the whole, though, Lichtenstein's Pop pictures could not be described as tragic. I asked him about one of my own favourites, a 1963 painting called Drowning Girl in which she weeps as her thought-bubble defiantly exclaims: "I don't care! I'd rather sink than call Brad for help!" When I quoted those words to Lichtenstein, he laughed and insisted that the sinking girl "didn't drown. I've always felt optimistic. I don't know why. I know it would be much more interesting if I wasn't. I've never done an anguished painting. I don't think about how I feel: I'm very even. Something terrible can happen in my life, but I wouldn't put it in my art. Is that the right yellow – that's all I'm thinking."
In a 1963 interview, "What is Pop Art?", Lichtenstein was at pains to stress that "the formal statement in my work will become clearer in time". I asked him what he meant by this prediction, and his answer was unequivocal. "I think what distinguishes good from bad painting is the formal statement, the position of marks and contrasts," he explained. "If you forget that I'm trying to depict a table, a window or some flowers, abstract qualities to do with size and positioning become important. That's what Mondrian was doing."
So perhaps Lichtenstein's art was more abstract than anybody realised. Music played while we talked, and it clearly nourished him. Lichtenstein told me that he had recently started playing the saxophone, and even maintained with a grin that "what I really want to do is music, but I won't give up my day job!" Although his tone was self-mocking, I noticed that the studio was stacked with CDs. And in the catalogue of his recent exhibition at the Venice Biennale, his one-sentence statement revealed: "I'm trying to make paintings like giant musical chords, with a polyphony of colours that is nuts but works. Like [Thelonious] Monk or Stravinsky."
He seemed one of the least puffed-up artists I had ever encountered, and his serenity was doubtless enhanced by his decision to spend only a couple of days each week in Manhattan. The rest of the time he worked in rural seclusion, in a place he described as "right near the ocean, too seductive to believe". Lichtenstein lived there with his wife Dorothy, but continued to think of himself as a New Yorker, and loved the life of the streets, even confessing to me that "I nearly killed myself roller-blading round here the other day".
Sadly, Lichtenstein's death, so soon after this interview, prevented the world from seeing how he would have developed in the new century. The loss is immeasurable, but the coming Tate exhibition will reveal just how much he did achieve.
'Lichtenstein: A Retrospective', Tate Modern, London SE1, 21 Feb to 27 May
Richard Cork's The Healing Presence of Art: A History of Western Art in Hospitals is published by Yale