Alasdair Gray scratches his goatee and shrieks, then adjusts the legs of his two-tone glasses. The sheet of paper he is looking for is one of hundreds on a table in the front room of his Glasgow west-end flat.

It belongs, like many others, to the as-yet unpublished A Life In Pictures, a comprehensive collection of the artist's work, or the novelist's art, for those who still see Gray primarily as the author of Lanark. It will be finished next year, he says, or maybe the year after that. His hands sink wrist-deep into scattered manuscript.

A discoloured skull stares out from a bookshelf, conveying a sense of finality that seems at odds with the rest of the room. Ageing, unfinished nudes line the wooden floor; incomplete limbs slowly atrophy on forgotten canvases.

"I feel my life as a painter has been full of not quite realised projects," says Gray, with a sense of drama, as he strikes upon the needle in a haystack. The partly-crumpled illustration shows a dapper Gray with the playwright and poet Liz Lochhead in 1972 – part of the credit sequence for a film that never happened.

Now And Then, had it ever materialised, was meant to showcase the work of both artists: the paintings of Gray alongside Lochhead's specially written poems. That those works are to be partially reunited – and in Gray's case shown as a series for the first time - during the Glasgow International Festival Of Contemporary Art is the source of excitement. And concern.

"One or two of the paintings were bought," he says. "Some I gave to friends who had been generous to me. But they haven't all come back. One was bought by a very fine set designer for Scottish Opera, who then lost it. Over the years there are pictures of mine that have been lost or stolen or strayed, and whose departure I deeply regret."

The 10-minute film was the brainchild of BBC Scotland's Malcolm Hossick, who had produced a couple of Gray's plays for BBC schools television. The plot would revolve around a solitary female character. "The idea was that she would be mooning around and doing things in a state of loneliness, remembering past times with her ex-lover," says Gray.

"When the film moved into flashback, the past would be represented through my paintings and a set of poems written by Liz Lochhead for the soundtrack." There was no budget, no deadline, and no end to Gray's immersion in the project. And then there were Hossick's unrealistic expectations. "He thought I could sketch people quite quickly," says Gray, "which I can, but by the time I'd finished the paintings he'd retired to Corfu with his wife."

His laughter is cut short by the arrival of Liz Lochhead, and then starts again just as abrubtly. There is no kiss on the cheek between the two, no shaking of hands - a quiet smile, a flash of eye contact, a years-old friendship taken for granted. Gray's wife Morag ushers the pair towards a set of mismatched, high-backed chairs, some of which have the magical realist properties of Gray's finest prose. "I'm scared of that one," says Morag.

"It's likely to swallow you up." Gray begins rocking in the rocking chair while the silver-haired Lochhead pulls off her coat. She has a terrible memory, she says, and little recollection of the early 1970s, but the Credit Panel illustration on the table jogs her memory. "My Afghan coat," she says. "That was the nicest Afghan coat in Glasgow. Most people's were sort of hard and smelly leather but mine was really soft suede and gorgeous."

Someone brought it back from Turkey, she says, but she can't remember who. She is the younger of the two, at 60 years to Gray's 73.

When they began working on Now And Then, Lochhead was 25, an aspiring poet and still unpublished. Gray was 38 and already a well-known name to Lochhead and her peers at the Glasgow School Of Art. He had studied mural painting at the school in the 1950s before reluctantly becoming an art teacher.

Lochhead ploughed the same path on graduating from the school in 1970, but not before seeing one of Gray's dramas brought to life. "I remember being in the audience at the art school for a performance of Alasdair's play The Fall Of Kelvin Walker," she says. "It must have been around 1967. When Alasdair walked in someone whispered, That's Alasdair Gray, he's a genius.' Lanark wasn't published at that stage but he was writing it, and everyone knew he was writing it."

Gray rocks in his chair, forward into the present moment, and then back to 1971. "I remember when Liz and I met," he says. "I was going through to Edinburgh on the train with a friend for a show at the Talbot Rice Gallery, and Liz was going to receive an award from Radio Scotland."

"I was really excited," says Lochhead. "The poets George Mackay Brown and Pete Morgan had judged two of my poems first and second place in this competition. How I had the nerve to go up and speak to you I don't know – I must have been high as a kite."

"You suggested I come along to meet them afterwards," recalls Gray.

"A convivial drink," says Lochhead. "We got the train back together and after that we were great friends. You would take me to see things like Shakespeare at the university. I remember going to see Twelfth Night. I would sometimes come and sit for you because you were painting that beautiful painting The Birth Of The Northern Venus."

"Oh yes," says Gray. "Which I still haven't finished."

Leonardo da Vinci's observation that art is never finished, only abandoned, seems particularly apt for Gray. Even when his work is abandoned, it will sometimes return or resurface like a stray cat. One of the paintings in the forthcoming exhibition, begun in 1972 for the film sequence, was only completed last year, thanks to the sharp eye of the writer James Kelman, a long-standing friend of both Gray and Lochhead.

"I had so many unfinished paintings that I decided many years ago to put some of them out," says Gray. "I suggested to my lodger at the time, the artist Carol Rhodes, that she and her art-school friends could use them as surfaces to paint on. A year or so later, Jim Kelman called me. He had been looking out his back window and saw what he thought was a painting of mine in a midden, in a state of some disrepair.

"My friend Angela Milan, who owns two of the other works from the series, heard about this and nagged me to restore and finish that painting for her. I only got it back from Jim's house last year and had it finished just in time for Christmas."

The painting – The New Room – shows the film's proposed female character played by Lochhead's then flatmate Doreen Tavendale, and her future husband Russell Logan, in Malcolm Hossick's flat on Glasgow's Great Western Road. Logan stands next to a baby-grand piano, looking pensively at the stars outside the window; Tavendale looks into the middle distance beyond the viewer.

"I remember that piano," says Lochhead.

"And the idol on top of it," says Gray. "That was Javanese."

"Doreen was very pretty," says Liz.

"Oh, you were all right," says Gray.

Lochhead and Gray had been friends for less than a year by the time of the collaboration. They were pulled close by common interests, says Gray, and the fact they "each somewhat admired the other's product".

That they both attended Philip Hobsbaum's now legendary creative writing workshops at Glasgow University, where they met and made friends with Kelman, Jeff Torrington and the poet Tom Leonard, only strengthened the bond.

"It was an exciting time to be in Glasgow," says Lochhead. "It was fantastic for me to meet all these grown-ups who were maybe 10 or so years older than me. Getting to know other people who wrote, and them taking me seriously as a writer, was exhilarating. In terms of my writing it was probably vital."

At that time, Gray was wading through the aftermath of separation from his first wife Inge Sorenson, lodging with friends and trying to maintain a steady relationship with his son, Andrew. "There'd been a considerable rupture domestically," he says, "and I was therefore pleased to make new friends. I remember visiting Liz and Doreen in their flat by the art school."

He slips into a stern professorial voice, which suggests something weighty is coming. "I'm afraid there was no sexual connection between us, at least not of the sensuous erotic variety, but I was platonically pleased and excited by the friendship of young, intelligent women."

Lochhead laughs. Gray's influence on her fledgeling career, she says, was significant. "I remember Alasdair got an Arts Council grant, which must have been at the start of 1972, and actually paid to have somebody type my poems up."

"It wasn't expensive," says Gray.

"It was a lovely present," insists Lochhead. "I think it led directly to the publication of my first book, Memo For Spring. When the publisher came and asked if I had any more poems to show them I already had those ones ready and typed out." The next poems she wrote, for Now And Then, had a further bearing on her future course, and that of contemporary Scottish theatre.

Without the aborted film, plays such as 1989's Mary Queen Of Scots Got Her Head Chopped Off might never have happened. Her translation of Moliere's Tartuffe into Scots may have been stalled. The midlife crisis suffered by Susan Love, the protagonist of her 2004 comedy Good Things, might have run a little deeper.

"When I look back now, the poems I wrote for Now And Then were really important because they pointed me in a new direction," she says. "They pushed me towards drama, although it wasn't really drama. But there was a dramatic aspect to the poems, a sort of story."

The pair lean into the glow from the electric fire, hunch over a reproduction of Gray's painting, Rainbow. It depicts what Gray refers to as an "erotic breakfast on a Sunday morning". A couple –Tavendale and Logan – laze idly on a floor, half-dressed, in front of a painting of themselves nearly kissing; behind that first painting is another in which they are, in fact, kissing; while outside the window a perfect rainbow forms.

Lochhead's poem to accompany the painting, also called Rainbow, adds further life and texture to the work: "as I watched your lips move/ (and fell more and more in love)/ and waited for a convenient lull/ to put something a bit more in my line/ on the record player."

Another poem, The Legend Of The Sword & The Stone, accompanies Gray's painting But I Dare You, which shows the fictional couple of the film at the point at which their relationship starts to crumble. "She's basically saying, I'm not happy with you but go on and see if you can make me happy'," says Gray.

Painted in profile, the bearded, shirtless man tries to engage his partner in an embrace. "They look like the couple in The Joy Of Sex," says Lochhead. "You never realise how much of the spirit of the time is going into what you're doing when you're doing it."

Gray laughs like he is full of helium and rocks wildly back and forth on his chair. His reputation as a visual artist will be strengthened by the forthcoming exhibition, which seems like timely, if not overdue, recognition. If the art has played second fiddle to his writing over the years, it's no reflection on the art.

His distinctive style means his paintings could be picked from a police line-up of hundreds, and he has been credited with inspiring a new generation of visual artists. Likewise, in the fields of theatre and poetry, Lochhead's pedigree is secure. The pair's friendship has endured, says Gray, for several reasons, but largely because they both live in the west end.

"I would say the main ingredient to any friendship is to live roughly in the same place or district," he says. "I had very good friends who are still friends, but they moved to Fife long ago."

"It's true," says Lochhead. "Alasdair and I have lived more or less within a square mile of each other for ... oh ... ages." She frowns, knits her brow. "I'm just realising that all my friends have been friends for 36 years or more," she adds. "That's terrible."

Not terrible that those friendships have endured, one imagines, but that so much time has slipped through the cracks. They look sternly at each other, pulling carefully at the loose threads of their memories. What has been lost is lost for good, unless one of them can recover it quickly.

Neither knows how much filming, if any, was done by Hossick for the Now And Then project. "I have a feeling he filmed a sequence in which Doreen was applying some make-up and dolling herself up a bit," says Gray.

"There's probably a can of film somewhere," suggests Lochhead. "Did I not do a voice-over?"

"No-one else would have done it," says Gray. "If we ever got that far. I don't recall saying goodbye to Malcolm when he left for Corfu." "When did he leave?" asks Lochhead.

"I don't even remember," says Gray.

Only three of Lochhead's poems – versus nine of Gray's paintings – are likely to have been found by the time of the exhibition. She remembers one – Snakes And Ladders – but doesn't know where it is. Or rather, she has an inkling but no idea how to find it. "It's somewhere in my archive, which is to say somewhere in a box," she says.

"Someone will have a photocopy of it, but I don't know who." Other poems written around the same time, she says, would have been equally fitting for the film. "It was the kind of thing that I wrote about at the time," she says. "Unhappy love affairs were my favourite subject."

Both sigh. The rocking chair stills. Gray stands and drops a chequebook, picks it up. It gets caught in the papers he is carrying to the pile by the window. There is no attempt to replace the pages in sequence in the manuscript.

He turns on his heels, straightens his tank top. Lochhead stands too, and says she has to rush. "Alasdair getting all of his paintings together makes me think I should do the same," she says. "I must get stuff found."

"Well, everything comes easy," says Gray, "when one has proper time for it."

First published in The Sunday Herald, 2008.

Paul Dalgarno

Paul Dalgarno

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Paul Dalgarno

Alasdair Gray and Liz Lochhead: making up for lost time