Kelly's heroics: How the British writer behind TV's most expensive drama cracked LA

ACGI-packed cross between Avatar and Jurassic Park, about a human colony that travels back 180 million years in time from a polluted and dying 22nd-century Earth to the age of the dinosaurs, the Steven Spielberg-produced Terra Nova is the most expensive TV series ever made. Each minute of the 13-part, £60m series has been estimated to cost £200,000, with floods on the set in Queensland, Australia, and the firing (and then re-hiring) of the show's writers adding to the price tag. Overnight reviews from Monday's US premiere have deemed the finished product "promising", which will come as relief to Spielberg, Fox and the show's creator, Kelly Marcel.

Kelly who? Well, Marcel is an actress-turned-writer from south London, whose credits until Spielberg came knocking included a BBC Scotland sitcom that never got made and some script-doctoring on the prison film Bronson.

And, frankly, Marcel is as surprised as anyone that a film-maker has spent as much as a dime on her script, originally titled Gondwanaland Highway. "I really didn't think it would ever get made because I thought it was just way too expensive," she says. "I just thought it would be a sweet little thing that I would give to my dad and that would be the last anyone ever heard of it."

"Dad" is producer-director Terry Marcel, most famous for writing the cult 1980 sword'n'sorcery movie Hawk the Slayer, and every Sunday Kelly goes round for lunch with Terry, whom Marcel calls a "walking Trivial Pursuit". "He was telling me about how the world used to be one giant land mass called Gondwanaland," says Marcel, explaining her original title and the genesis of Terra Nova. "Also he was reading one of Stephen Hawking's new books about time travel, and I had just seen An Inconvenient Truth [the 2006 documentary about Al Gore's mission to educate Americans about global warming], and so I just put those three things together."

Marcel, in the midst of boxing up her belongings for a move to a new house in leafy Twickenham ("Terra Nova paid for my deposit") quickly penned a 15-page treatment outlining her characters, the world they were living in and a story arc for the first season – as well as a 30-page "bible", which summarised how the show would pan out over five seasons. Gondwanaland Highway was soon picked up by Carnival Films, the makers of Downton Abbey. Marcel was poised to sign a deal with them when her future American co-producer, Aaron Kaplan, persuaded her to stay her hand.

"He called me up and was like 'please don't sign that contract... please don't sign it... you've got to bring this show to America'. And I was literally throwing up about not taking the deal I had in England."

The decision not to sign with Carnival seems even tougher when you consider that Marcel then had to pay her own way to Los Angeles and hawk the script around the major networks. "I brought on another writer at this point," she says. "An American writer called Craig Silverstein who knew how pitching works. Pitching's quite a skill – you go and you've got 15 minutes and you have to kind of perform the show for them."

In the event, a bidding war broke out between two of the networks, Marcel finally going with Fox. Sent home to England to write the two-hour pilot episode, she was utterly amazed to receive a phone call one day saying that

Steven Spielberg was reading the script. "I was kind of whoa... what... who? That's bonkers. The man who made ET is reading my words? How in the hell is that possible? But they said 'they're looking to him to produce it'. Hollywood is full of mad things and I thought that that was just another madness. And then about a week after that they called up and said, 'Yeah... Steven's going to produce your show'."

And Spielberg did – a very hands-on producer, too, apparently, although Marcel is yet to meet the great man. "No, I never met him," she says. "All of our conversations go through the network. I'm sure we'll meet in due course but when he was in London doing War Horse I was in LA doing a Ben Stiller film and then he went back and, to be honest, when I decided not to do the show anymore it became Steven's show – I handed it over. So he's got no reason to meet me really."

Marcel was asked to stay on the show and be a guiding hand in the way of Matthew Weiner on Mad Men or (more pertinently in this case) J J Abrams on Lost – but, as she says, she turned the job down. Why? "Only because when I went out to LA to pitch Terra Nova I very cheekily decided I could pitch a second show as well," she says. "Having been told that nobody ever, ever, ever sells two shows, I pitched Terra Nova in the first week and sold that, and then sold Westbridge to Showtime in the second week."

Westbridge, set in small-town Texas and following the lives of six workers on Death Row at the local prison, "is more the kind of thing that I like," says Marcel, who, along with West Wing producer Tommy Schlamme, flew to Texas in the middle of writing Terra Nova to hang out in local penitentiaries. She was supposed to go and watch an execution, but that didn't work out. "Although I was in the chamber a couple of hours after it happened, speaking to the people who had tied him up... it was a pretty intense experience. And because I'd had that experience, when it came time to decide between Terra Nova and Westbridge, I went for Westbridge. We still don't know what's happening with it, but in the meantime I've got lots of movies so that's great..."

The movies include Ben Stiller's new one ("a dark comedy, it's a very different character for Ben"), and she's just finished Saving Mr Banks for Ruby Films, the company owned by Lily Allen's mother, Alison Owen, and which made Shaun of the Dead, Tamara Drewe and the new Jane Eyre. "It's about P L Travers, who wrote the Mary Poppins books, and her relationship with Walt Disney over a three-week period in which Disney invited her to LA to try and get the script for Mary Poppins right. She was a tyrannical old cow who absolutely didn't want that film to be made... You can imagine one of our dames playing the curmudgeonly battle axe..."

She's also busy adapting a modern-dress version of the Greek tragedy Medea for the BBC. "I'm killing four children as we speak," she says cheerily. "And in January I do a book adaptation called Mr Chartwell, which is an amazing book about Winston Churchill's depression. He called it his 'black dog' and so the lead character in the film will be a six foot-seven black dog..."

It's an extraordinarily productive time for someone who spent her first five years as a writer unable to get anything made. Marcel started out as a child actor – an early role being in her father's 1978 horror flick Prey ("I was the prey... I was eaten in the first five minutes"), while she spent two of her early teenage years at the RSC and the National Theatre, part of the reason why she left school after her GCSEs. Quitting acting aged 25 (she's now 37), she worked in a video shop, started writing and script doctoring and, after an emergency rewrite on the movie Bronson, made a firm friend of the film's star, Tom Hardy. Indeed Hardy has a tattoo dedicated to Marcel.

"It takes up the entire inside of the top half of his right arm," she says. "He is such a sweetheart – people who mean something in his life he feels the need to indelibly ink on his body." Marcel and Hardy formed a theatre company, Bad Dog (yet to produce anything, since they're currently both so busy – Hardy filming Christopher Nolan's new Batman film, The Dark Knight Rises), as well as penning two comedy dramas together, both of which were bought by Channel 4.

Little wonder then that Marcel, who has not had more than two days off in two years, has had to postpone her wedding three times. Actor Thom Fell, her fiancée, recently joined her at the scripting game. "He was kind of, 'ah, that writing thing looks kind of good, I think I'll give that a shot', and I was 'go on then, because it's only taken me seven years...' Literally six months later he had sold his first show – to ITV; it's called The State of Dave. It's a brilliant idea, it's brilliantly executed and I hate him."

'Terra Nova' starts on Sky1 at 8pm on Monday 3 October