Artist Steven Campbell is not himself. It's not that he's out of sorts. You can lay to rest the speculation about his health that accompanied Campbell's first Scottish show in nine years at the Talbot Rice Gallery in 2002. He looks better: plumper, flamboyantly dressed and his painting seems healthier than it has been in a long time.

It's not that. It's that he's someone else, someone playing the artist Steven Campbell, perhaps. Or that the author of his new pictures, in an exhibition entitled Jean-Pierre Leaud, opening today at Glasgow Print Studio, might not be Campbell. Campbell claims he's been sharing the work with a character he calls the invisible man or, since he has been working among the esoteric carvings of Rosslyn Chapel in Midlothian, the green man.

As we walk around the studio, he'll point out an odd bit of painting, fake and lurid colours in the middle of a complex canvas. ''The green man became a character in this area of the canvas,'' he says in his characteristic tone, somewhere between a growl and whisper. What Campbell is doing, in a spectacular return to form, is playing the game of the artistic alter ego. Marcel Duchamp had his own female double named Rrose Selavy; Turner used to sneak about the streets under the name of Admiral Puggy Booth.

For Campbell, who has always had an interest in theatricality, in authorship, in double deals and disguises, and who produced performances as well as paintings in his days as a student at Glasgow School of Art, a doppelganger lets him run amok. He is also tapping into one of the most persistent themes of Scottish literature. In visiting Rosslyn, he also revisited his interest in Robert Louis Stevenson, whose Dr Jekyll and Mr Hyde became the most famous double act of them all.

''Stevenson used to visit the chapel. I started thinking about him, he's so much up my street. When I'm painting, I'm someone and when I'm not painting I'm someone else. I never know who I'm going to be.''

Brilliant, with dazzling painterly and verbal dexterity, Campbell is the finest Scottish painter of his generation. Emerging from art school in 1982, he won a Fulbright, and made his name in New York. In a rather lazy bit of pigeon-holing, he was later lumped together with other Glasgow graduates such as Ken Currie and Peter Howson, under the heading of the new Glasgow Boys or an apparent return to expressionism, but the connection did him a great disservice.

The son of a steelworker, who followed in his father's footsteps before going to art school, Campbell is a classic autodidact. His work is

rich with references, not just to the history of painting, but to obscure philosophers, avant-garde films and odd bits of scholarship and natural history. It is dangerous, though always tempting, to read the work as an expression of the life. Living in Kippen, Stirlingshire, with his wife, Carole, Campbell remains ever-ready to emulate and slyly mock the romantic image of the artist.

He is a dandy who has named his current show for the iconic French actor, Jean-Pierre Leaud, whose career began by playing a character not unlike the director Truffaut in

his film 400 Blows and reached maturity playing a character of a

film director under the director Bertolucci in Last Tango in Paris.

It's a question of stand-ins, of substitutes, of true fakes. It's also a question of apprenticeship. Leaud was a director's apprentice - he kept up a career-long relationship with Truffaut, even though their politics and their lifestyles were poles apart.

In this show, Campbell, at 51, reveals his own long apprenticeship in the world of painting. His new works refer to Magritte and to Cezanne. Somewhere among their complex imagery you can see Titian's Allegory of Prudence or an iconic flag painting by Jasper Johns.

''In these paintings,'' explains Campbell, ''there are some very obvious references, such as Bosch or Cezanne, and quite a few other paintings. I've never ever put anyone else's painting into mine before and,

when I did that, I felt like I was being directed a wee bit, like in a film. I was becoming the third party, rather than the first or second. The painting became distant.''

If that sounds complicated, that's simply because Campbell's painterly world is complicated. It is a world where appearances are deceptive, where man and animal might morph into one another, where danger lies hidden around every corner, where the underworld and the real world slip and collide.

In the Prisoner's Invitational Run, a group of men disguised as animals, straight out of nightmare painting by Brueghel or Bosch, run across the picture plane to freedom. In Mistress Wife Grandmother of Jekyll and Hyde, a beautiful woman - based on Monet's portrait of his wife - drags figures from the dark underworld beneath her skirts. In Jekyll and the Hydes a group of three young men, streetwise boys straight off the streets, leer confrontationally at the viewer; between them they have 12 heads.

Campbell's last show was ragged. It dealt with death, detective stories and violence beneath a veneer of suburban normality, but it was also terribly raw. It was hard to distinguish Campbell the consummate play-actor, the mischievous fan of detective novels, from a man who might have been in real trouble. When a TV documentary about him was shown on BBC Scotland, his frail appearance rang alarm bells. He received worried calls from friends.

With this current work, though, Campbell seems freer. He still likes to talk about emotions and describes painting as a ''struggle against desperation''. He still seems terribly dissatisfied with his work, but he also has an air of detachment, a sense of the comedy, intelligence and sheer love of painting that has always marked him out. He resists all my suggestions that his current paintings reflect a man who feels freer, or happier, but whatever Campbell may say, his work is stronger, more analytical, than it has been in a long time.

In work, though, and in the complex web of characters, real and imaginary, that stock his world, Campbell may come closest to happiness. ''The artist Neil Dallas Brown once said that it's the only way a man can order his life, by painting, by working. Everything else is disorder and nonsense, out of control,'' he says. ''I've never forgotten that. With painting I love it but I hate it. It's god-sent, it's just the way it is.''

Steven Campbell, Jean-Pierre Leaud, is at Glasgow Print Studio until September 25.