David Hockney stands beside the minimal pond and slab of decorative wild flowers outside the Tate’s Clore wing assuming the characteristic pose of the anonymous smoker outside large buildings all over the West. Unconsciously, smokers collect themselves into aesthetic tableaux that mimic the background extras of our culture. They look like the choruses in baroque opera, Shakespeare’s attendant lords, the youths lounging in high-Renaissance skulduggery.

There is no smoke without ire, and Hockney is plain furious. The hot tip of his indignation grows livid. He draws a deep breath and contemplates. Dressed in a bespoke pinstripe suit and a lightweight white flat cap that I think is a touch cricket umpire but he claimed to be California golf, and on second glance I reckon he’s more Hovis baker than either of those, he could have stepped out of one of his own opera sets. He always wears a hat, he says, because his eyes are so sensitive – like Betty Grable’s legs, they ought to be insured for a million.

Hockney is the most enduring British artist. With his owlish face, with the character spectacles and Just William hair, he is the most recognisable painter in Britain. He’s been up there in the visual charts for four decades, the Cliff Richard of art. These days he also wears discreet hearing aids, and his soft Yorkshire accent has been roughed up by the fags. Indeed, they’ve colonised his conversation. “I’ve been disenfranchised. My vote counts for nothing. I’m going to be made a criminal because of this.” He holds up a stub. “And I hate Gordon Brown, I really do hate him. I’m going to be made a criminal because I smoke. They’re taking away our liberties and there’s no one saying a thing.

“Did you see my piece in The Guardian? That passive smoking is nonsense… And cancer. They don’t know the causes of cancer… Anyway, it’s my life. I’ve smoked, it’s my choice… I was stopped by some girls in the park. They told me off for lighting up. I told them, I said…” Hockney smoulders brightly on in the sunny morning, reeking pleasantly of kippered academician. We’re supposed to be here to talk about Turner’s watercolours, but we have to get the ciggie break out of the way first. He’s not quite obsessed by it, but he’s certainly addicted, hooked on the unfairness of a ban and his purloined rights. Inside the Tate, up a staircase, are the Prints and Drawings Rooms, not open to the public without an appointment. Here they keep part of the huge Turner bequest, the watercolours, notebooks, letters, sketches, engravings. The conservators have put out two sketchbooks for us to look at. Hockney begins turning the pages and we talk about them in a distracted way, like an old couple watching television, but soon drift into a mesmerised, softly exclamatory silence. The pages turn, revealing slow tracking shots of clouds and horizons, the lightness of touch. The tentative but wholly assured pencil darts over the paper, leading you on. The brush drags and skitters, sometimes retracing itself, pausing, then skipping off, running out of pigment so the stroke disappears into a frayed edge, like a flicked fringe. Our eyes follow, inexpert trackers, seeing as Turner did without seeing what Turner saw.

Here is a bucolic scene of shepherds moving their flock through a wood, and over the page, the same sheep seen from the same place have moved further down the road. You can make a guess at how fast the sheep are travelling. I don’t expect 19th-century sheep moved any quicker or slower than 21st-century ones, so you can sort of work out approximately how long it took Turner to draw. It’s electric, faster than it’s taken me to write it. He must have done it barely looking at the paper, touch-drawing.

He was an inveterate, compulsive recorder of everything that passed in front of him; sunsets and clouds rush across the page, costermongers and labourers lounge for a moment in corners. Sometimes the lines are such a pared-down shorthand that you can only hazard a guess at what he was looking at. The sketchbooks are as intimate and revealing as diaries. “I don’t think Turner used a camera,” Hockney says thoughtfully, breaking the reverie. No, I don’t expect he did. “He’ll have known about photography, of course.” Of course, yes. “But I don’t think he used one. I can always tell.” Really.

Cameras are the other subject possessing him. This isn’t an addiction, but it is an obsession. He’s written books about the use of cameras in art, made TV series, composed hundreds of letters. It’s controversial, though Hockney says he’s satisfied he’s won the argument that many of the world’s greatest artists secretly used photographs to inspire their work. But then that may be because the rest of the art world has agreed not to mention it any more. Like Richard Dadd, the painter who killed his father and lived in Bedlam for much of his life, calm and sane unless someone mentioned God and then his eyes would roll and the attendants would reach for the straitjacket. Cameras are Hockney’s God. And he has a lot more to say about them and it takes a lot of time, so I’m not going to mention it all here. I’m not saying he’s wrong, I’m not saying anything, but it does come round a lot, like the last vol-au-vent at a wedding reception.

The curators have also put out a selection of watercolours that Hockney has chosen to be at the heart of an exhibition, Hockney on Turner Watercolours, which started last Monday. Why did you choose these ones, I ask. “Well, they’re just the ones that caught my eye. There isn’t any particular theme.” Except that these ones are all unfinished? “Yes, I like to see the marks, how he worked – they are fascinating.” Perhaps more so for artists and critics. They’re like taking the back off a watch to see how it ticks, or perhaps even more, like reading music.

Turner made marks on paper that are more like notes of music than any other watercolour artist. His oil paintings are like symphonies with an orchestra, but the watercolours are violin solos. In a few of the pictures Hockney’s chosen, you can see how Turner has lost the image, where the marks and the paint have run away from him. He goes back to rescue them, to resolve something from the mess. All these unfinished images come to us like this because either they were in some way unsuccessful, or whatever it was Turner needed to learn from them was accomplished. So you can look at them as either successes or failures, or as images that pirouette between both. Drawing is for an artist as much about the process as the result. With watercolour you travel from pristine light to muddied dark. Although the aim is to reveal, the practice is to cover up. Why do you think Turner was so engrossed in landscape? “He was fascinated with the surface of the world. He never got tired of just looking at it.” That may be the case, but I think it’s truer of Hockney than it is of Turner. Hockney talks a lot about how people don’t see any more. “They don’t stop to look at what’s around them. Their eyes are blurred and focused on TVs and computers and, of course, cameras and photographs.

Tourists only look through viewfinders. I never carry a camera.” But you used to. You used to take lots of snaps and stick them all together. “That was different.” Why Turner was so fascinated by landscapes and recorded them so constantly all his life is an enigma. They weren’t particularly fashionable or considered high art: that was the plinth occupied by historical, allegorical and classical painting, and Turner did plenty of that. He was a barber’s son from Covent Garden, with a cockney accent. He wanted to get on, to be recognised, to be accepted and, more importantly, to make money.

When landscape was used by the painters of the late 18th century, it was invariably as a metaphor for some human emotion, or as a portentous indicator of the meaning of a scene in the foreground. Very few artists went outside to look at a landscape and paint it. Topography was composite, invented, a metaphysical place. The purpose of civilisation, of which fine art was in the vanguard, was to tame nature, turn it into a garden, a park, a simile. And watercolours were particularly parochially English, the preserve of lady hobbyists, illustrators, cartographers, botanists and amateur military anthropologists.

Unlike his contemporary Constable, there was never a Turner country. It wasn’t about recording a landscape that represented home or a sense of belonging. Turner was an inveterate traveller. Every summer he’d go off on difficult, exhausting journeys. There was something in mountains and extremes of weather that spoke to him; the symbolic meaning of the landscapes

are camouflaged by his peerless manipulation of brushes and colour. As Hockney says, the miraculous surface of the work is so seductive, but it is also just as obvious that there is more than the reproduction of a place, or the weather report of a particular day. These pictures worked like music. As with his contemporary Beethoven, under the brilliant construction of the compositions there is a beat that speaks of freedom. These mountains and vistas, the rolling sea, are evocations of the new century’s big idea: individual, innate liberty. As records of journeys, they encapsulate the right to go and see places, to meet people. The metaphor in Turner is in the act of just going. These things, the mountains, the horizons, look back at you and mirror your individual liberty, the freedom to go and think and say and be. The landscape isn’t a simile of a single theatrical emotion or a deific purpose, it is altogether an explanation of the towering reason and the liberty of the human spirit. The pictures, however small and delicate in scale, open out into great airy deep-breath views.

Everything about Turner’s eye and sensibility looks out, across, over and beyond. His divisions don’t imitate the omniscience of gods, they are the observations of the highest calling of humanism. People inhabit his landscape as occasional devices to lead the eye, give scale. We’re not expected to empathise with them. We identify with the landscape, the horizon is our journey, the mountain the possibility of our achievement

Turner holds a singular and charmed affection in the public eye. His pictures are wholly visible and understandable, whilst eliciting thoughts and ideas, rather than feelings, that are shrouded and profound and lyrically moving.

There is an exhibition that explains some of Turner’s techniques. There’s a map showing how his palette changed as he moved south towards the sun. And a time line of pigment manufacture. His life coincided with the age of professional manufacture of paint and paper. You can see where the deep eggy yellows come in and splash across the surface of his pictures.

There are tables that are arranged so that the public can sit and copy his drawings. Hockney starts to draw a church steeple. I sit down to a typical group of everyday folk, the smokers that Turner often sketches into the margins of his pictures. There is no swifter or surer way to understand the methods and mood of an artist than by copying him. It’s not about producing a good likeness but going over the motions. It is a linear deconstruction. Turner draws with small, nervous but precise lines. He was trained as an architectural perspective draughtsman. Later he uses the pencil like a small brush, caressing outlines, making rapid little strokes that are deliberate but tentatively questing like a dog’s nose searching a hedge for a scent. After 10 minutes, Hockney’s drawing looks like a Turner steeple. Unfortunately, so does mine.

Outside for another cigarette break, Hockney mentions that his father was a passionate anti-smoker years before it was medically or socially fashionable. “He took me to Aldermaston wearing a handmade badge of a diseased lung. It didn’t really look like a lung, just a brown thing with a black spot.” Hold on, your dad was a passionate anti-smoker and a bad painter. You don’t think that your frankly eccentric devotion to cigarettes might have something to do with him? Hockney thinks about it for a moment. “No.”

That afternoon I go to see his latest painting at the Royal Academy. It takes up one entire wall of the best naturally lit room in the place. It’s of a wood, painted in oils, on square canvases fitted together like a quilt. It’s vast. “I couldn’t have done that without a computer,” he confides. “No one could.” No, I don’t expect they could. After a morning with Turner, I’m bereft of words. Wow, I hear myself say. And we stand there again, sharing the silence of an old couple in front of an Imax screen, thinking perhaps that individual liberty and freedom of expression have, in a century and a half, led to Hockney painting a life-size computer-assisted copse in Piccadilly and me to ogle it in astonishment. Hockney’s Turners started last Monday. You can see the greatest achievements of our greatest artist, which are rarely put on public display. You may also see Hockney. He’ll be outside fuming s

BP Exhibition: Hockney on Turner Watercolours is at Tate Britain until February 2008. Tel: 020 7887 8888

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