Joanna Briscoe: How my novel was brought to life

At first it sounded like a dream come true – her bestselling novel was to be turned into a primetime drama by the biggest scriptwriter in the business. But from the moment she saw her characters step on to the set as a cast of actors, Joanna Briscoe realised that the whole experience would be far more weird and wonderful than she could have imagined

Imagine a world in which your thoughts can be read. And misread. And made prettier, darker or more unsavoury. This alarming place is inhabited by strangers loudly broadcasting desires and fears and neurotic little complexes you once believed were yours alone – life material transformed into fiction during innumerable hours of solitary labour in a landslide of a study, but somehow still privately imagined until now. There's a Tannoy on the side of your head and it's booming out your fantasies. That's what it feels like watching one's novel being turned into a film.

"But – but, it was someone else who had that awkward peck of a kiss," I want to say. "And that was six years ago in a different square." I have never felt so bewilderingly in charge of puppet strings yet entirely irrelevant as I watch hundreds of people converge to create a new commodity out of what was once just a product of my mind.

In 2005, my novel Sleep With Me was published by Bloomsbury. Andrew Davies – the scriptwriter behind the wet-shirted Colin Firth version of Pride and Prejudice, Bleak House and the recent Sense and Sensibility – bought the novel, read it, knew that he wanted to adapt it, and approached his regular script editor, Ellie Wood, who was now head of development at the independent television company Clerkenwell Films. I started talking to Ellie when there was little more than a rumour of interest: a rumour that was so enticing, I refused to dwell on it until there was some evidence.

The rumour blossomed. Deal details were emailed back and forth between Clerkenwell's company director Murray Ferguson and my agent. We had a riotous working lunch, as lunches with Andrew Davies tend to be, then Andrew went off and wrote a draft, worked with Ellie Wood's notes for a couple of further drafts, and produced a final script that ITV commissioned as a two-hour drama.

Between rumour and first day of filming, more than two years had gone by. Unlike publishing, in which an accepted manuscript means a book on the shelves roughly a year later, television is a battlefield of proposals, speculation and dying projects. Film is even more precarious.

But now, in the late spring of 2008, shooting is actually starting. I walk down the hill from Hampstead to Chalk Farm in north London expecting something: excitable tourists, autograph hunters, some delightful token that this is a red-letter day. Nothing. I continue. Still nothing. I walk on, round a slight bend, and there before me glints the evidence: massive lights ranked on the pavement outside Marine Ices, where the first day's filming is taking place. I actually feel tears in my eyes.

Films are almost always shot out of sequence, and this initial couple of scenes starts with a pivotal moment in a much-loved Italian restaurant where my three protagonists converge. The place is strewn with cables, lights, microphones and smoke machines used merely to diffuse the glare of the lights mounted outside to reproduce hazy winter sunlight. Extras loll around getting hot and playing on their mobiles.

"You'll get Ross Kemp and Amanda Holden as leads and you'll have to pretend you're pleased," one cynical director told me. Mournfully, I lowered my expectations. However, Murray Ferguson and his team had other ideas. Adrian Lester (Hustle, Primary Colors, Peter Brook's Hamlet and the National's Henry V) was chosen to play the inappropriately fixated Richard; his wife Lelia is played by Jodhi May (The Last of the Mohicans, Tipping the Velvet and Daniel Deronda), and Anamaria Marinca (Four Months, Three Weeks and Two Days and Sex Traffic) is the outsider Sylvie: a seemingly drab interloper, a cuckoo in the nest of a settled relationship.

This is one classy cast, and I was thrilled. I had to do a quick visual reshuffle in my mind. Richard, originally white, was now black; Lelia, originally half-Indian, was now white; Sylvie, originally French, was now Romanian. The real adjustment of my own dreamed-up templates occurred the moment the actors opened their mouths in the script read-through and a new version sprang into life. Yet at heart it remains just what it was: an intense psychological drama about character and desire so, mercifully, I haven't had to experience the soul-destroying sensation of watching my story turn into something unrecognisable.

Now, at Marine Ices, sound and lighting trucks parked down a blocked-off side street, everyone busies themselves before the actors arrive. Fascinated, I photograph everything I can. Crew are setting up rigging (scaffolding and platforms), lights and sound; make-up artists are at the ready and a medic is on hand – such frenetic activity necessary before the actors, aka the "artists", arrive. The people sitting in the restaurant are not normal diners but extras who will be paid a day rate. But extras, these days, are not extras. No no no. They are "Supporting Artists" or "SA"s. If they speak, they get paid more; if they say more than a certain number of words, they're classed as an actor and given a contract.

A clapper loader fills in the clapperboard, which shows the scene and take number; the principals arrive, fully made-up in winter clothes though it's a warm spring outside, and the atmosphere changes. There they are, those characters: once mine, then Andrew Davies's, now the product of a huge collective effort; there they are in their shiny new incarnations, bizarre and hyper-real after the softer sketches in my head. There is a hush, a change of focus, a feeling of the air in the room tightening as the attention turns to them, while passers-by gawp through the windows and teenagers loiter on the pavement outside for the entire afternoon. The actors move straight into rehearsal, snapping into character with full concentration. I have a new understanding of the craft of this profession.

"From the very top then, guys. Turning..." calls the first assistant director. Silence descends, all tiny sounds magnified, breathing audible. "Action..." A large, traditional film camera is being used since Sleep With Me, unusually, is being shot on 35mm to achieve a depth and quality impossible on other formats. "Cut," calls director Marc Jobst.

I'm peering in infantile excitement round the restaurant's alcoves to catch the scene where Jodhi May and Adrian Lester talk as they eat ice cream. I know teenage boys who would kill for a glimpse of their hero from Hustle; middle-aged women who become misty-eyed about his Shakespeare performances. "First positions, thank you very much," calls the first AD. Smoke is waved around. Anamaria Marinca is sitting reading at another table, her character with her subtle magnetism about to infiltrate the lives of others. She's like a dream in the filtered light in her uptight little coat – specially made as winter coats can't be bought at this time of year – and almost worryingly like the Sylvie who existed in my imagination. I can tell that the costume designers have read my novel. I stare, mesmerised and slightly uncomfortable, feeling that mind-reading sensation again.

The same two scenes of a few lines each are rehearsed and then shot time and time again with absolute concentration. I've got the whole film vibe wrong: I'd blithely imagined a party atmosphere in which I could chat with the actors between takes, have my kids along, bring groups of friends willing to help out as extras crashing on to the set. Far from it. This is a workplace. The actors are in character between takes and sealed off from milling civilians by their concentration and that acquired I-am-immune-to-all-stares skin that somehow separates them. It's a force field one wouldn't want to penetrate. As for the kids and friends on set, health and safety and insurance issues rear up: all visitors must be accounted for, and given permission to be there. Strangers are ushered away by security.

As the shoot gets under way, it takes on a life of its own, with camaraderie, a unique and rarefied atmosphere, a set of rivalries and flirtations and running jokes springing into life. The crew start to refer to the film as "Feet With Me", a joke about what they perceive to be the director's obsession as he lingeringly focuses on bare feet walking over stone and wood.

For those intense weeks, it's boarding school; it's Big Brother; it's the office party. "It becomes your life," says Anamaria Marinca. On-set breakfast is at seven. We leap upon the tea urn and old-fashioned biscuits in the morning break; the catering van with its queues and steaming puddings on unchippable crockery appears to serve school dinners that we eat at tables in an old double-decker bus surrounded by loud chat. "The vibe of the set is so much brighter than the vibe of the real world," says Rick Stroud, a seasoned TV director friend. "It can be hard to adjust when it's ended."

I can see already that it will be difficult to leave this behind. I watch as one of the teenaged girls employed to play young versions of the characters sobs as she hugs director Marc Jobst goodbye after her two days of shooting. "I always take it home with me," says Marinca. "I don't think there's any other way to be."

Even as an extraneous presence, as both outsider and originator dipping into it, I absorb that extraordinary, intoxicating atmosphere. "It's all about capturing magic," Stroud says. Filming is so slow, a few minutes or even seconds of footage shot in a day, that comparisons with watching paint dry abound, and indeed there are dead periods, repetition, and endless waiting. Yet there's nothing quite like it. "It's been a clockwork shoot so far," says the producer, Steve Lightfoot. "The clock's a bit slow sometimes," he jokes. "But it's been a joy." Ellie Wood says: "The actors were really challenged as every line can be nuanced – a lot of TV work isn't as interesting and demanding on the actors."

"It's a happy shoot, an intense shoot," says Marc Jobst. "It's a real challenge – this is proper acting. Because the characters have hidden lives, it's subtext-driven, whereas most TV drama is dialogue-driven and is cut, cut, cut. This has a European sensibility – there are whole layers of story going on underneath and that's why the actors are loving it and working hard. I wanted actors who would demand that the camera stay with them. This is a three-hander really, so it's got to be an intense psychological thriller about the psychology of infidelity." Murray Ferguson, who is executive producer, agrees: "Much of what is significant in this is mysterious, elusive and unspoken."

Sleep With Me is all shot on location in London. If the shoot is longer, or takes place abroad with cast and crew thrown together in hotels, this is when, say my on-set sources, "the shagging takes place, and you find out who's an alcoholic, who's bad-tempered, who's great, and who's not. At the beginning, people think they're best friends. By the end, they can be mortal enemies. They all love and hate each other. You get all the sex-in-the-hotel stories and then everyone's standing by the monitor the next day and you enquire casually, 'Sleep well?'"

My next visit is to a square in Bloomsbury where the shoot is taking place. It's suddenly an alarmingly cold spring day; crew huddle in hats; there's a cluster of booms and lights and people in fluorescent security jackets around the actors. Again, all focus is on that spot. Murray Ferguson makes a set visit. Richard and Sylvie's tentative kiss is filmed as imminent snow is discussed and seed fluff from the plane trees falls in abundance instead. What we perceive as simple daylight on screen is aided by massive lights, panels, screens and reflectors. The kiss and dialogue are repeated eight times before the director is satisfied.

The aspect we viewers never consider is the months of planning before a shoot can even begin. A pre-production office is set up with full-time staff some weeks before the shoot, and then a location manager with a location scout begins the process of finding suitable locations for all the scenes, asking permission, organising contracts and payments, speaking to the council and police for filming and parking permissions. Costume designers and art departments have to get to work, and building and painting permission has to be sought from buildings' owners, while the "unit base", consisting of the trailers, make-up vans, admin office, drivers, portable loos and catering vans, has to be set up as near to the shoot as possible. "It's both practical and creative," says the location manager for Sleep With Me, Mark Gladwin, "which is what I like about the job".

On to Gordon Square in Bloomsbury for a five-day chunk of the shoot. A towering Georgian house belonging to the University of London is used for the first time ever for filming. The place is uninhabited, yet I assume someone's been there for years as I step inside the warm and lived-in maroon clutter of what is Richard and Lelia's flat. This is an incredibly convincing illusion: an empty room has been "dressed" from scratch with a shabby bohemian profusion of books and pictures, the details including photos of Adrian and Jodhi looking younger, while a false wall, an entire kitchen and upstairs bedrooms have been constructed. Why not film in a real flat? There's usually simply not room for all the equipment and crew.

What we perceive as one staircase leading to an attic room is created by joining together three different clips. Sylvie leads Richard up a smart staircase, filmed in Willesden, that leads to an attic staircase, filmed in Gordon Square, that leads to an attic, filmed in Highgate. For each scene they have the same clothes and make-up, the three shots are joined seamlessly, and we assume it's filmed in the same place.

What I had also failed to understand is that a two-hour television film requires the same crew and working process as a feature film. Hundreds of people work on this production. As I wait at the next location, Templeton House in Roehampton – a stately home in which a wedding scene will be filmed – I watch the rigging going up outside windows to beam in massive lights, dozens of people in high-visibility jackets walking around. This is a different world with its own language: a world of grips and hod riggers, gaffers and dollies.

Crew run around with radios and earpieces; a first, second and third assistant director busy themselves; a gaffer works as the right-hand man to the director of photography, or DOP, while a "best boy" is assistant to the gaffer; chippies, sparks and a unit manager are on hand; a focus puller measures distances between the artists and the camera and then sets the focus; the grip looks after the dolly (a moveable platform for the camera), and pushes it along tracks for a moving or "tracking" shot. Fizzy apple juice stands in for champagne and is brought in by props assistants, the glasses constantly refreshed for new takes. Plastic tape is put on the floor to mark up where actors will walk and stand.

Jodhi is standing there immaculately made-up in a beautiful wedding dress, but wearing Converse trainers underneath as her feet won't be shot. It is fascinating to watch the mechanics of the illusion: the extras mouthing dialogue, the gruesome dolls that stand in for babies in buggies, the tricks of lighting. Cosmetics alone can create a narrative: the character Sylvie, initially understated, begins to take on the bloom of Lelia with more blusher and lipstick as the incursion into her life progresses.

On to Highgate. We're at Witanhurst, London's largest house after Buckingham Palace and an incredible, decaying, listed sprawl in five acres of tumbling layered garden that feels as though it must surely be in the middle of the countryside. Uninhabited for years, it's where Fame Academy was filmed. So vast is it, I become lost on separate staircases, and run into painters decorating rooms in other wings for the forthcoming filming of Shangri La. Certain rooms are sealed off because of asbestos: the security instructions for this day alone run to 12 pages. Some rooms have mysteriously taken on an entirely French flavour for Sleep With Me. I climb on to the roof and stare at London spreading beneath me and feel lucky to have access to amazing places I wouldn't have seen if it hadn't been for this film. "Here we go then," calls the first AD downstairs. "Nice and quiet... stand by then. Turn over, please." "Turning," echoes the third AD. "And action," says the first AD. The camera rolls.

"You don't have the time to prepare," Anamaria Marinca says. "You have to risk everything in film. It's on the moment. It's seizing the moment. It's about adrenalin, it's about being able to dive into emotion when you have to... We're blind painters; we don't see ourselves. We don't see our work. Directors have a bit of a distance; I need someone to guide me as an actor."

Andrew Davies arrives and we do press interviews. He always makes me laugh; we have a roaringly good time, giggling and muttering asides to each other. Then we're interviewed together for the Sleep With Me DVD that will be on sale after the broadcast. He and I go out to lunch at the local Café Rouge with Ellie Wood, Marc Jobst and Steve Lightfoot; again Andrew has us all laughing. Two tables along sit our principals, eschewing the catering van. I want to get my head around this bizarre profession of theirs. It's fascinating spying on them interacting in real life and watching their altered dynamic. I'm floating between perceptions of what's real and what's not; of true memory and distorted memory; of where my fictional version bleeds into someone else's.

The day always starts early, with much pre-dawn construction, the actors collected in cars between six and seven in the morning and delivered to make-up and costume. They work a 12-hour day, six days a week. "Filming is a complex mix of pleasure and stress," says Murray Ferguson. To my slight disappointment, I witness no thespian hissy fits, no actresses storming out of Bloomsbury squares. But when a shoot is working, when there are no hitches of the bomb scare variety, it's a beautiful thing to watch – a symphony, all separate elements falling into place in an intricate pattern of activity as everyone applies their own skills yet works towards the same end.

As I see builders and electricians wandering about whistling as they prepare for the shoot, it strikes me as utterly surreal that all these people are employed because of imaginary characters in an imaginary setting once created in my head. Having been able to watch a new three-dimensional version evolve has been one of the most intense and fascinating periods I can remember. I feel I've been let into a closed world and I feel privileged for it. "This is my life for a month," says Anamaria Marinca. I feel the same. It's an unreal life, but an addictively highly coloured one.

Sleep With Me will be screened on ITV later this year