If anyone can, Ken can

Sally Hibbin is just half an hour's drive from Glasgow airport, where the December morning was bright and crisp and clear, and already a dense mist is snaking around her rented car. The mist passes eventually, as the ever- narrowing road dips and rises vertiginously, winding into the mountains. But it will return that afternoon, more obtrusively, to descend upon the set of a new film which she is producing and the director Ken Loach is shooting in the area. The film, provisionally titled Carla's Song, is set in 1987, and traces the love affair between George, a Glaswegian bus driver, and Carla, a young Nicaraguan woman, as he accompanies her back to a Nicaragua that is being ripped apart by the Contras.

Hibbin hasn't worked with Loach since Riff-Raff in 1991; despite the fact that it's 8am and Carla's Song is less than a fortnight into a punishing three-month schedule, her enthusiasm is obvious. So has Loach changed much in the time they've worked together?

"I think he's mellowing a bit," she suggests. "He doesn't usually allow journalists on set." There is something else, almost an afterthought: "Ken doesn't like the actors talking to the press - they've got a media ban until after the shoot." Of course.

Those actors are Robert Carlyle, of Hamish Macbeth and Trainspotting, who has had anything but a media ban over the past month, and the Nicaraguan newcomer Oyanka Cabezas. Casting for the film, Loach discovered Cabezas in her home country; she had acted and danced professionally a few times but until six months before shooting began on Carla's Song, she couldn't speak a word of English. Once he met Cabezas, Loach and the writer Paul Laverty did some subtle engineering work on the screenplay in order to accommodate her linguistic inexperience. On set, she is quiet and sweet- natured, given to communicating with crew members through a shrug or a smile, listening attentively as Carlyle offers her a bit of local history, and only becoming sprightly when she's larking around with her friend, or Laverty, who speaks Spanish. She is utterly disarming and very beautiful; people gravitate toward her.

By 9.30am, most of the crew have completed the steep 20-minute climb to the site of the shoot, overlooking the yawning grey expanse of Loch Lomond. Today, they will tackle a scene early in the script where Carlyle commandeers an orange bus into the mountains for a picnic with Cabezas. The scenes with the bus are to be filmed nearer the loch; first will come the more simple set-ups following the couple as they make their way on foot. I catch sight of Loach, striding across the land with a small posse tagging along behind him. With his blue windcheater and bright red scarf, he has the unassuming look of a rambler taking in his morning stroll.

He offers a cheery, breathless greeting, then bounds off down the mountainside to prepare Carlyle and Cabezas for their shot. We all dive to the ground, out of the actors' field of vision, out of sight, holding our breath, not daring to move. Crouching behind a rock, I glance up at the horizon. A ginger sheep is loitering in the distance, quizzically observing us. I can see its breath hanging in the air.

Loach holds his camera back as far as possible to diffuse the situation's artificiality as much as possible; the actors need to forget the illusion and forget who they are (hence the media ban). To this extent, Loach is unusual in that he relies heavily on improvisation and shoots scenes in strictly chronological order. There's a flashback in Carla's Song, but rather than film that scene when the whole production is scheduled to move to Nicaragua a few weeks later, he has already flown out there and completed it. "I can't imagine doing things any other way," he tells me later. "Because as you go on, you can never tell what will develop between the actors. You can't capture that same progression if you don't film chronologically."

When the shot is through, I notice Laverty, nosing around the crew-members with a video camera that BBC Scotland have loaned him - they've commissioned him to film a documentary about the making of Carla's Song. He's a small, mousey man given to talking quietly but with wild enthusiasm. He has been on the set for each of the 11 days so far, and he's buzzing from it. "They probably think I'm a pain in the arse," he chuckles, nodding at the crew, "'cos I've got the attention span of a gnat."

Laverty's script is wrung directly from his own experience. He was working as a lawyer in Glasgow in the early 1980s when he decided that he needed a bit of an adventure, "the way you do when you reach your twenties". He ended up in Nicaragua, doing human rights work, just as the country was discovering what it meant to be on the sharp end of US foreign policy. "I witnessed low-intensity war," he whispers, "and a systematic attempt to tear this beautiful country apart. I had to deal with cases of murder and torture - it was stuff that would make your blood boil. I just went back there a few weeks ago and really it's just an impoverished Latin American country like any other. They've got the highest per capita world debt in history, and they'll never break out of it. That's what the US has done."

How do you go from being disturbed by a situation to writing a screenplay about it?

"I think the film was just born out of massive frustration," he explains. "And a need to remember. Milan Kundera said, `The struggle against power is the struggle of memory against forgetting'. I think there's great truth in that." We stand savouring the pure air, which is so crisp that you can crunch it between your teeth. That thick mist is creeping up from the loch - it has already engulfed the land below us. The ginger sheep is fussing and fidgeting on its ledge; it has clearly never experienced the tedium of life on a film set before.

Robert Carlyle plucks a cigarette from his pocket as the crew breaks for sandwiches and soup. I've been wary of Hibbins's warning all day but it's finally Loach who introduces me to Carlyle, though he lingers around us like an over-protective chaperone wary of his ward being wooed. Carlyle grills me about Trainspotting - with two months until it opens, he hasn't seen it yet - and excitedly begins to describe some of the more graphic scenes to Loach. The pair first worked together on Riff-Raff. Is it easy adjusting to Loach's style of working again?

"Well, he's the... "

"Um, look," Loach interrupts, "maybe we'd better leave it there." Carlyle smirks, his unfinished sentence bursting to leave his mouth. He looks like a schoolboy whose teacher has just told him to button it. When Loach scurries off to organise the next set-up, the actor leans in to me and, jerking a thumb at his director, simply assures me that Loach is "THE MAN". When Loach returns, he offers a completely unnecessary apology.

"I'd rather he didn't talk to anyone," he explains patiently, "because then he'll start thinking about things in terms of technique and who he really is, and it'll all become conscious." Point taken.

As the camera prepares to roll again, a plane begins winding above us in broad circles, its engine always frustratingly within earshot. And no sooner has it vanished again than the mist is tearing towards us, obscuring the horizon and making continuity impossible. We wait for 10 minutes, watching the air thicken, watching the light change.

But the mist has swallowed the sheep, and the mountains, and any hope of continuing. Finally, Loach calls it a day - the scenes with the bus will have to be postponed until tomorrow ("though I haven't allowed for that in the schedule," he confides, as though surrendering a guilty secret). As the equipment is being loaded into tractor trailers, someone chirps, "We've only got four shots in the can". "Yes," Loach retorts, "but they're good shots". His smile tells you that there's nothing to worry about. But just to be on the safe side, he has a last request for his crew: "I want everyone to get on their knees tonight and pray for tomorrow's weather." It's the least they can do.