Three teenage misfits cross paths in Glasgow, agree that music is their salvation, and decide to form a band. Sound familiar, Belle and Sebastian fans? That's because it comes directly from the mind of that band's frontman, Stuart Murdoch. God Help the Girl is the 45-year-old's writing and directorial debut. It was developed from an album of the same name in 2009, which he wrote with the intention of having mostly unknown female singers provide the vocals.

The genesis of the film stretches back to several years before the album was conceived, however. Considering its prolonged gestation period, is it fair to characterise it as a labour of love?

“The thing is, when you say ‘labour of love’, it sounds like I’ve been slaving away, but nothing could be further from the truth,” he chuckles, speaking from the “wee flat” in Glasgow that he uses as his office, where he wrote the film’s script. “I feel that I’ve served an apprenticeship; I wrote my first script, I wrote my first songs for other people, I wrote my first musical and became a director – and a producer as well, because I had to raise the money and all that stuff – so all of this was like having a dissertation in film-making. And dissertations sometimes take five or six years, so I feel armed and ready.”

Murdoch has fronted Belle and Sebastian since they formed in 1996. Despite directing the odd music video for friends, including Camera Obscura, a film career “was never a burning ambition”, he says. “But I’ve always found in life that you’ve always got to wait for that feeling that you can’t say no to: it just comes up and sweeps you along. And that’s what happened with [the main character] Eve and the film.”

Less daunting than an album

The idea of writing a script was less daunting than writing an album. “I would say that the four or five months that I wrote that script were possibly the happiest period of my life, in a kind of selfish way,” he says. “It was the first time that the band had downed tools for any significant period of time – this was in 2006 – because we’d been busy for 10 years. It was the first time that I could sit at a desk and let my imagination run riot.

“The scripting was new to me. At first I found it very easy, because to me, it was just people talking inside my head, and I wasn’t too concerned with plot. I just let them flock and ramble, those three characters. So I had a great time: to be honest, I didn’t work very hard. I would do an hour in the morning, and then I would spend a long time on my bicycle, or just out walking.”

Casting the film’s three main characters – Eve, a fragile young girl who escapes a psychiatric ward where she is receiving treatment for an eating disorder; James, a charmingly neurotic misfit musician who falls for her; and Cassie, their sweetly ingenuous friend – was the most crucial part of the process, says Murdoch.

"When [producer] Barry Mendel told me that casting for this film in particular was going to be so important, I didn't take him that seriously," he says. "And then when we finally got into it, it was a crazy rollercoaster. They tell me that we saw 1,500 people for Eve alone, which is nuts. It pretty much took a year, but it was so important to find a good bunch. And each of them took the character in a certain way. With a film like this, you have to be so flexible, because if you're not, you're going to get something stilted, so we were constantly writing and rewriting. They weren't meant to be two English and an Australian – far from it, they were meant to be two Scots and an American – but we had to go with the talent."

There is a distinct Murdoch hue to the film, not least in the musical set-pieces, which are intended to be the songs written by the characters as their musical set-up progresses.

“It’d be a bit daft if here you are, making your first feature film, and you say, ‘Actually, I’m going to do everything that’s against my instincts’,” says Murdoch. “Of course it is a Belle and Sebastian song come to life; it is a Belle and Sebastian album cover come to life. I think that’s fair enough for your first feature. So nobody could have really directed it apart from me, because I wanted every frame to illustrate what the songs were saying.”

He specifically avoided watching musical films while writing the script, although others from his childhood and teenage years played a role: there are bits of everything from Withnail and I to Gregory's Girl and from Pretty in Pink to Fast Times at Ridgemont High in there.

“I don’t much care for musical films, [so] there were even more dramatic films that were benchmarks for this film,” he says. “I love those films, and you can learn something from them. Why do they stand up? Why are they real? Why do they actually speak to you at the time? There I was, late-30s, early-40s, and sitting down to write this film about youth, or my youth, or a youth that never happened. But I still wanted people to find something to connect to, and love in it.”

It started with a song

Murdoch says having songs to base the story around made the whole process a lot easier. "I'm not a great storyteller and I don't have this great urge to write a story, so it wasn't plot-driven in that way. Instead, I had the songs and I had to hang it around the songs. But once you've written the songs and imagined in your head what would happen, that's quite a big chunk of film that you've already got. And the bonus with writing songs is that quite a lot of it is subconscious. I didn't sit down and say, 'I've got to give Cassie a song here; what would she sing now?' or 'What would Eve sing at this point?' There's a song called The Psychiatrist Is In that Eve sings to James. I remember being off on my bicycle, cycling to Edinburgh, and I had that tune in my head. Every mile or so, I'd stop to write more words down, and it was just kind of flowing. It wasn't like a jigsaw puzzle that was part of the script, but I thought, that's a good song, that'll go right in the middle of the film."

Next stop, Jessica Alba?

The steep learning curve hasn’t turned him off the idea of making more films. “It’s funny: I sat down with some New York agents after [the film premiered at] Sundance, and they said, ‘You have to make your next film straight away. You have to move fast’, and stuff like that. I just kind of laughed at them, and thought to myself, my God, I’m quite happy to go back to the group. But they were saying, ‘Oh, we could hook you up with Jessica Alba. She’s got a new project.’ She seems like a very nice woman, but I’m not going to make a film with her, unless I can write and direct it.

“I’m sure any future films I do would be authored by myself, and that takes a bit of time. I don’t feel that I have the authority to direct someone else’s work.”

For now, when he is not busy with his one-year-old son, he is concentrating on finishing Belle and Sebastian’s ninth studio album, the bulk of which is recorded. “It may be the very start of next year when [it] comes out,” he says. Promotional duties for the film are planned for the autumn, including a planned stop-off in Dublin in October.

He already has an idea for a film based around his "amazingly patient, amazingly supportive" Belle and Sebastian bandmates, most of whom have cameos in God Help the Girl. "I've said to them that at some point I'd like to maybe do another musical film with another writer/director and take the band into it. Obviously I don't have another 10 years to throw away, but I can see how things can be a lot faster and better now that I've done it. Of course, the cat's out of the bag now and I'd love to make another one.

“They’re dreams,” he says, “they’re just bigger dreams than songs. And they’re a massive indulgence, but who doesn’t want to indulge them?”

The Irish premiere of God Help the Girl is at the Galway Film Fleadh on Saturday. galwayfilmfleadh.com