This is the heavy, heavy monster sound! (At least, we hope it is)

When Madness asked Tim Firth to write a musical, he was the first to raise an eyebrow...

For me, the musical constitutes the best and worst that theatre can be; the most transporting articulation of a story or a sausage-string of tunes joined by a series of thin twists. As a kid of 13 I sat in the eighth row of Evita and decided I wanted to write. My early influences were musicals, not plays: Superstar, Blood Brothers. Yet you couldn't drag me to sit through Kismet or that one with Tommy Steele as a leprechaun. I hate what can happen to dialogue when it approaches the vortex pull of an upcoming song. "Ah I see you've brought your little dog with you. I used to have a dog. Little fella. God I miss him. Let me tell you...". Cue song about dead dog. My gravest concern in writing a musical based round the songs of Madness – an undeniably cart-before-horse situation – is to avoid the sausage-string.

For me, the musical constitutes the best and worst that theatre can be; the most transporting articulation of a story or a sausage-string of tunes joined by a series of thin twists. As a kid of 13 I sat in the eighth row of Evita and decided I wanted to write. My early influences were musicals, not plays: Superstar, Blood Brothers. Yet you couldn't drag me to sit through Kismet or that one with Tommy Steele as a leprechaun. I hate what can happen to dialogue when it approaches the vortex pull of an upcoming song. "Ah I see you've brought your little dog with you. I used to have a dog. Little fella. God I miss him. Let me tell you...". Cue song about dead dog. My gravest concern in writing a musical based round the songs of Madness – an undeniably cart-before-horse situation – is to avoid the sausage-string.

To rewind. A retrospective diary up to this point:

December 2001

See Madness in concert as a punter.

January 2002

Asked if I'd be interested in collaborating with them. Pros: the songs are endemically theatrical. Cons: the phrase "the latest in a line of ...". Having said that, I can't be snobby. Most musicals are based on pre-existing stories. This is just the reverse.

A story that merits a musical is rare. They have to be truffle-like: small, uncomplicated, pungent. The genetic components of Madness are wit, emotional honesty, irreverence and a muscular sense of place. I can't get out of my head a scenario where the car in "Driving In My Car" is "not quite a Jaguar" and then a flip to where it is exactly that. A story is starting to emerge of a life lived two ways.

Potential directors. First choice, Matthew Warchus. I tell him how I'm not interested in doing a "fly past" of songs so passionately that I realise I'm shouting slightly and he's moved his chair back.

And now MW and I are in the Tricycle Theatre workshopping a story about a young Camden lad who commits a petty crime to impress his girl. The musical follows the two paths his life would've taken had he done the right or wrong thing that night. Rob the designer has just pointed out that by weird coincidence the Northern Line splits in two at Camden Town.

Later in January

We've cast the lead boy. Michael Jibson hasn't even finished at drama school. I love the way he diffidently tries to suggest a girl he's at college with for the female lead.

February

Case the Cambridge Theatre with Phil Bateman the MD. The audience is largely Dutch. I'm sitting there thinking, "How the hell are these guys going to understand a split story?". To make things worse, as soon as the leading lady starts singing a ballad, an entire row of Canadian students get up to have a beer in the stalls bar. A frightening night.

March

Still no lead girl. Problem is that there's loads of great West End singers out there, but Madness are not "West End". If someone comes on and sings "My Girl's Mad at Me" in the same way they sing "Bring Him Home" then the audience will be straight out of the theatre, and I'll be with them. No vibrato. That's the key. In performance, set, costumes, every aspect of production – no vibrato.

April

Get a call from Matthew. He thinks he's found our leading lady. I travel down from Cheshire to see Julia Gay, the girl Michael suggested from his college.

July

Read through. The cast appears to be entirely from the North. Told this to Suggs who said it's perverse that this musical tells more about the history of the band than their biographies, because it's the story of all their own childhoods.

August

I was told very early on that musicals aren't written, they're rewritten. We've already arrived at the comedy ritual of "Tim's homework" where everyone sits in a semicircle with pencils poised. I have to feel unrepentant about this. In a musical, a line of underscore can suddenly render pages of dialogue redundant. An explosive dance affects everything in its wake.

Later in August

We're not going to hit the first preview date. The show is more of a movie on stage than anyone expected. MW tells me he is going to petition the producers for an extra week of previews. This would entail moving back the opening night a week and cost a fortune.

The final run-through in the rehearsal rooms goes surprisingly well and immediately after it MW petitions for the extra time. He gets it. The producers say that tonight they got their first glimpse of what the show could be. They talk of not spoiling the ship for a ha'p'orth of tar.

4 October

Dress rehearsal. For the past two weeks, the cast have worked on each scene in isolation. Running them together at speed, people are vomiting with exhaustion in the stage-right toilet. Suddenly, way up in the fly roof, the massive, descending Act One set crashes. Cast flee into the auditorium.

8 October – First preview

I can't believe the audience are in; the last time I saw the production the cast were running for cover. I sit in the eighth row with a thousand people around me and realise that for half this cast the last show they did was at drama school.

9 October, 1.40am

As I watched them cope tonight with all the teething problems, I just felt immensely proud of them. It helped that the audience had gone bananas by the end of the first number. Their confidence grew and it will grow more. The script will tighten. My original aim was that people who don't know Madness would think these songs had always been written for a musical. I don't know if I've achieved that. But on leaving the theatre the band tell me they're more proud of this show than of their first number one. So that's something.

'Our House': Cambridge Theatre, London WC2 (0870 890 1102), previewing, opens 28 Oct, booking to March 2003