It's funny, but a trend has spread itself ever so slightly across Edinburgh over the past few years, whereby once wackier-than-thou, gag-a-minute purveyors of stand-up routines in search of an audience are putting pen, pencil, and brain-cells together, sitting down, and going legit. Because, despite the annual whine that comedy has spread itself across town like a particularly voluminous rash acquired after hours on the Cowgate, what's actually happening is the complete reverse.

Funny men and women are going straight, learning lines, and acting in or writing plays of their own devising. It's all a far cry from the laughter-hungry bear-pits of foolish youth which now form a burgeoning circuit, while studio and pub theatres across the land rack their brains on how to get bums on seats.

Just look at Jim Sweeney, not so much a comic as an improv impresario, a regular member of London's Comedy Store Players, who, with the likes of Steve Steen and Stephen Frost, has indulged in a series of off-the-cuff theatre games, once strictly the property of what used to be called the alternative comedy scene, but which later trickled down into mainstream TV via Whose Line Is It Anyway?.

A couple of years ago, Sweeney's first play, Danny's Wake, was something of a sleeper hit on the Fringe. The tale is of two old mates, a pair of likely lads on the verge of going to seed, brought back together for a holy reunion at the funeral of a third. This year, Sweeney's long overdue follow-up, Sick Transit, extends the theme, by having three old muckers in a clapped-out band on the pub-toilet touring circuit huddled together in a service station waiting for their broken-down van to be repaired.

"It's Danny's Wake without the coffin," Sweeney points out matter of factly a couple of hours before he's due on stage at The Comedy Store. "It's literally just three men talking, and could really be set anywhere."

Not for Sweeney the let's-make-this-precious approach to a very private and all-too-rarefied craft often shrouded in mystique. But then, "I'm not a writer," he insists perhaps a tad too forcefully, "and I would never ever describe myself as such. I don't have any of the disciplines that a writer has, but I have nothing but admiration for them, even though I didn't think much of them for years. Even so, it's not a world I would particularly want to inhabit, because it's a nightmare."

This isn't luvvie affectation, merely an acknowledgment of the loneliness of the long distance playwright. "Because most of the time I do improvisation," Sweeney points out, "you say something, and you know straight away whether it's funny or not from whether people laugh or not. With writing, you sit there in front of a computer screen and you write on your own, and you're like, 'Oh, I'll put this little word here, but what if I put that one over there? Oh, but I'm not sure if that makes it better or not,' and there's no-one to bounce it off. With this I couldn't think for ages what it was about, until one morning, it all filtered through properly, and it was stupid that something as simple as a van breaking down took so long.

"But writers, real writers, I totally admire, because I think they must be insane, lonely people. It's just horrible. I hate it. I do not enjoy the process one little bit."

Sweeney recently did a TV writing workshop, in which he found his fellow pen-pushers' social skills limited, to say the least. But then, neither is he overly enamoured with clock-watching actors in search of a tea break, and nor does he align himself with what he calls "born again improvisers", with their multi-coloured matching T-shirts and clichéd party tricks.

Which is why, clearly, he gravitated towards the altogether more sociable world he inhabits, full of daft onstage bonhomie and instant applause, and where, within five minutes of central London's offices closing down, there's already a bit of a queue forming outside The Comedy Store, where Sweeney chats amiably to regulars and wannabes.

So why, with all this 21st-century showbiz glamour at his fingertips, would he want to put himself through the living hell of writing something which he won't know is any good until it's already in the public domain and up for scrutiny?

"It's ego," he says candidly, "and no matter how horrible writing the thing has been, there's something about completing the script and handing it over, and people laugh or whatever; that makes you feel good, especially with Steve (Steen) and Steve (Frost), who've said every word as I've written them, so that any bad writing in there sticks out a mile, and I've nobody to blame but myself."

It was also, Sweeney freely admits, a game of call my bluff, after taking over an office from Improbable Theatre's Lee Simpson, who's in the director's chair for Sick Transit.

"I'd made excuses for years," he adds, "saying I couldn't write at home because of the kids and the phone always going, so now I had this office where I went in every morning at 10 o clock, had a cup of tea and a sandwich, and just wrote, because I knew I had to do something. It was a painful journey," he laughs, "but that's how Danny's Wake happened."

Given that Sweeney appears in Sick Transit alongside Steen, whom he's known since they were both 13 years old, and Frost, one might be tempted to see the play as a dissection of the long-term compadres' own friendship on the road to middle age. Sweeney, however, counters the suggestion. "There was something about the age we were when we were doing Danny's Wake," he says, "and somebody said it was nice to have two men onstage speaking so gently to one another. But I don't think we've all ever had a deep and meaningful discussion."

As we speak, Sweeney has been in day three of rehearsals, and has just talked himself into a rewrite, a luxury he wouldn't normally have on his everyday circuit. And, for all he might be in denial of what he may or may not be, he seems to be giving himself an unnecessarily tough time.

HE SAYS: "The others are fine, but there was just this huge leap of logic that needed changing, because I knew I hadn't put the original work in when I was supposed to. I suppose I just wanted to feel vaguely comfortably with it before we got to Edinburgh. The rest of it doesn't matter. But writing is a very scary thing for me, in the way that improv is for other people. There's some kind of perverse Catholic self-loathing that is satisfied by that, but ultimately it's all down to ego."

Sweeney recently had the weird pleasure of watching drama students perform a section of Danny's Wake as part of their final-year showcase. "It was so thrilling," he says, "and to hear people laughing, and know they were laughing at it rather than at Sweeney and Steen. Like I say, it's all ego." Spoken suspiciously, like it or not, like a writer.

Sick Transit, Gilded Balloon, until August 26, 4.30pm.

- Aug 9th