Stuart and I have been friends since our early twenties, when I was young enough to doubt all of his creative abilities.

I told him he'd need to write songs with a chorus if he wanted a hit. I thought busking in leopard-print trousers was as far as he'd get. Eight albums later, he's shooting a musical feature film in his favourite city.

I'm excited for him. I'm also enthused to get a day away from mothering two toddlers, and mostly I'm just nosey. I want to get in with the action.

By 7am, six other extras have arrived through the rain to the double-decker bus at "unit base". There are drama students, casual B&S (Belle & Sebastian) fans and a French girl on holiday. After a punchy fry-up breakfast we are hurried through wardrobe and driven to Kelvingrove Underground station.

We're guided through a series of walk-by scenes, swiping past the main actors who are having an argument on the street. The actors look youthful and kooky, as if drawn from graphic novels or plucked from early Woody Allen films.

The girl shouts, disbelieving, to the boy, "baby animals and cake, James?" and then stomps off.

My fellow extra, the French girl, speculates on this random snip of dialogue. I enjoy telling her, how, 20 years ago, when Stuart and I were not unlike the actors, he advertised for a flat share. He wrote wryly that applicants must be like "baby animals and cake". Later, a man with a voice like Quentin Crisp left him an answerphone message claiming to be an ideal flat mate as a lover of both.

I am surprised by my longing to be one of the crew. They seem important and filled with purpose. They look like they rarely moan and can find solutions to every problem.

I start to appreciate the language of the set. Confident voices shout phrases that become almost musical in repetition. Quiet everyone. Stand by. And rolling - At the end of each scene they check "the gate" and announce that "the gate is good". They could say "God is good" and I might be converted.

After lunch I'm called to makeup. Getting my cheeks brushed with powder and my hair straightened is no hardship. The afternoon involves a scene where a cute actress plays a dancing waitress serving lattes to a "worker, a management lady and a government spy". As "management lady" I get to wear a suit and high heels. I imagine I'm an exec, producing films like this. My role in the shot is to nod to the beat when my coffee is cleared away. I aim for a camp nod that isn't hammy. Hey, it's a fine line.

I consider then, that being an extra is one big self-conscious attempt to be unself-conscious. We know we're of lesser importance, yet we must take our role seriously. Our micro gestures mustn't jar. We can't glance at the camera like David Brent or wave like the French & Saunders background artistes.

And yet, there's a pleasing democracy at work. It's fun because everyone is pulling together and wholly focused on the shoot. No-one has time to stroke Facebook on a smart phone. We are strapped to the present in one big team effort of creativity.

At the end of the day I nod goodbye to Stuart, who is deep in post-wrap analysis with his Hollywood producer. And, once more, I am proud of him. The artistic verdict on the film is still open. Who knows if it'll be brilliant or awful; or if the back of my head will look good in a few shots? I'm just happy to be a part of it.