Kelly Marcel: 'Someone from Disney's going to come and kill me'

For a woman who spends her days in a shed, the Kelly Marcel story has a lot to pack in. Right now the name will ring few bells, but any account of her professional life must find room for Steven Spielberg, Holby City, Walt Disney and the tattoos of actor Tom Hardy. It would be nice to have a screenwriter on hand to knock it into shape – one like Marcel herself, among the most sought-after writers in Hollywood, working from the bottom of her south-west London garden.

Perhaps we should cut straight to the mention of what will surely be a landmark in her career, her script for the movie adaptation of Fifty Shades of Grey. Abruptly, the mood feels tense: "I don't want to talk about it," she says. "My work is done." Can I ask how it was to do that work, wrestling with the erotic favourite of 70 million readers? "It was ..." She hovers on the cusp of more. "No. I'm just not going to talk about it."

But that scene may give a misleading impression. At 39, Marcel is a slight and striking woman in a geometric print dress, bright, funny, good company. She would probably prefer to be elsewhere, but that doesn't seem personal. "I get freaked out doing interviews," she admits. "I just want to sit in my shed and write shit."

The reason she's here is Saving Mr Banks, a new film about the making of Mary Poppins, as squabbled over by its mastermind Walt Disney and the character's creator, novelist PL Travers. The stars are Tom Hanks and Emma Thompson, but the film belongs to Marcel – her script an expert confection, a masterclass in narrative structure. There is already talk of an Oscar.

Remarkably, it's her first film – though the situation is muddied by whether you count her uncredited role rewriting the 2008 prison biopic Bronson. Either way, soon afterwards she sold the idea for a sci-fi series called Terra Nova to American TV. Produced by Steven Spielberg, it became the most expensive TV show in history. An actual script sold too, a drama about Death Row called Westbridge that went unmade but became a highly efficient calling card among Hollywood executives. In the weird world of big-league scriptwriting, Marcel became a fearsomely hot property without a single on-screen credit.

Among the suitors was the British producer Alison Owen, who much to Marcel's surprise agreed to let her write the story of Travers' and Disney's fractious relationship. "Because obviously Disney would never let anyone make that film, least of all a bunch of women from London. But Alison said: 'Fuck it. Go on then.'"

More remarkable still, the movie was not just made – but by Disney itself. Marcel had assumed it wanted to buy the script simply to bury it. But she insists she was left free of corporate meddling. "I swear to God, I work for studios, and that happens. But not on this one."

In truth, Marcel's version of Uncle Walt is a long way from scandalous. Yet such is the studio's neurotic reputation that an official portrait of their founder slugging Scotch and displaying a steely self-interest feels genuinely bold. Perhaps a new era has dawned.

Until now, Marcel has been playing things straight, the veteran of LA meeting rooms. Then someone less on-message creeps into view. "I do think Walt might be frozen. I'd love to think he was." Does that mean she wasn't clued in while working on the film? "Yeah, they're really going to tell me where the frozen head is, aren't they? Now look – someone from Disney's going to come and kill me."

Marcel is the oldest daughter of the film director Terry Marcel, best known for the 1981 fantasy romp Hawk the Slayer. As a teenager, she went to the local state girls' school in Twickenham. Her reports there were "terrible – I wasn't nasty, I was just bored. I didn't care." At 15, fond of getting drunk on Richmond Common, she left school with no GCSEs. Her parents were convinced she was destined for shelf-stacking. Instead, she turned to acting. When I mention this she shudders. "I don't know what you're talking about. The internet is wrong."

Her first role actually came at the age of three – volunteered by her father to be eaten by an alien in no-budget horror movie Prey. In her teens and 20s came years of bit parts, Casualty and The Bill. It was a living – until, at the end of the 90s, she realised: "I completely hated acting and always had." The epiphany came on the set of Holby City, playing a character in childbirth, "huffing and puffing. And I got up afterwards and thought, 'Well, that was horrendous.' And I realised just how awful I found it. I couldn't ever be in front of a camera again."

At 25, a retired actor with no qualifications, she pondered her options. For all her boredom at school, she remembered loving English – and the example of her father, who had written as well as directed. She would, she decided, become a screenwriter. And so began a decade's worth of overnight success.

The first thing she co-wrote – a TV pilot about Alcoholics Anonymous – sold to BBC Scotland. It vanished into the limbo of development, but the memory sustained her during the dry spell to come in which her steadiest income came from working in Prime Time Video in Battersea. The job was perfect – in the dying days of video shops, the lack of customers allowed her to write in peace while the stock became a personal reference library. She screws up her nose at the idea of writing classes – she learned her trade watching episodes of The West Wing.

If Marcel's life were a movie, we would now be watching a montage of rejection letters. But a plot twist wasn't far away. Around the corner from the video shop was the Latchmere pub, home to a workshop for resting actors run by Tom Hardy. The two became friends. And when Hardy's movie Bronson "ran into trouble" while being directed by Nicolas Winding Refn, it was Marcel he summoned to deliver frantic nightly script rewrites. In gratitude, the actor added to his collection of tattoos the deliberately misspelled tribute "Skribe".

A career fell into place at last. Marcel produced a treatment for a TV show in which a 22nd century family were transported to prehistoric times "purely as an exercise in 'Can I write something my dad would like?'" Of course, it triggered more interest than anything she had ever written, her agent booking her a fortnight in LA pitching to American networks from out of a fleabag hotel. She sold both Westbridge and Terra Nova.

The offer for Terra Nova was to write a 13-episode season at $300,000 a show. Awesomely, Marcel said no. She was, she says, just unsure it was what she wanted to write. Spielberg's brainwave that the show should involve dinosaurs didn't help. Instead, she simply sold the idea, returning both to London and to Prime Time.

Her judgment proved sound. Terra Nova was cancelled after a single series. For Marcel, untainted, the offers of work became a deluge. Now her CV is still ever expanding. Already written are a film about Winston Churchill's depression and an untitled Ben Stiller movie. Westbridge is finally being made by the BBC. And then there's Fifty Shades of Grey …

Since being hired to bring EL James' kinky behemoth to the screen last year, Marcel's script has been the subject of rabid attention. Before now, there have been excited updates to the press, reported quotes that her screenplay was "going to be raunchy. We're 100% going there".

But she 100% isn't with me. "There just isn't anything to say." Really? "No." She's trying not to laugh. How full on is it? "In what way?" The obvious way. "I don't know. They haven't shot it, have they?" Just one quote, I say. "I'm sorry." Oh, go on. "I can't." And then she cracks: "I'm not allowed to talk about it! That's why it's so awkward. I've been very specifically told not to talk about it while I'm doing this." And right then the Disney PR steps into the room to end the interview. At the thought of Uncle Walt being associated with Christian Grey, the limits of Disney's perestroika would seem to have been reached.

I email Marcel later. I still don't get anywhere with Fifty Shades, but I ask her the most important lesson she's learned as a writer. "Learning which hill to die on," she writes back from the shed.