Pee-Wee Herman. Douglas Coupland. Sandra Bernhard. In-N-Out Burger. These were among the 49 Tweeters Diablo Cody was following in the last week of September. That is, when she wasn’t fielding both congratulations and commiserations from some of her own 100,000-plus followers. “Team Tara is jubilant. Team J-Bod is suicidal. Playing both sides is exhausting. Can't distinguish between whoops/wails on phone,” she Tweeted prettily.

Team Tara would be the cast and crew of the television drama series The United States of Tara, which Cody created, and which picked up an Emmy for its star, Toni Collette. The ceremony was just three days after the horror movie Jennifer’s Body, which Cody also wrote, opened to overwhelmingly savage reviews and dismal box office.

A few days earlier, when we’d been talking about her habit of spending Halloween at theme parks, Cody had confided that her real weakness is riding rollercoasters. “That’s what makes me happy,” she said. Just as well. Her life has turned into such a dizzying switchback ride, she should probably be suing somebody for whiplash.

On the plus side, she’s still crazy busy, what with Tara going into its second season, her regular column for Entertainment Weekly magazine and at least four screenplays in various stages of development, including an adaptation of the 1980s kid-lit staple Sweet Valley High and a modern makeover of The Taming of the Shrew for Steven Spielberg. Talking of staples, she has also been talking to Hugh Hefner about a Playboy movie — a mind-boggling prospect, to be sure.

But now there’s a sizeable debit on her account, too. Jennifer’s Body wasn’t an expensive movie to make, but inevitably it came with sky-high expectations, not least because its combination of the Transformers pin-up Megan Fox and the piping-hot Juno screenwriter promised so much. Between them, the two women’s previous two movies had grossed well over $500m (£305m) in America alone.

Jennifer’s Body mustered less than $7m on its all-important opening weekend, coming in fifth in the box-office charts behind Cloudy with a Chance of Meatballs, Love Happens, the latest from Tyler Perry, and even Steven Soderbergh’s box-office misfire, The Informant. The critics weren’t offering any consolation, either: “Well, the Diablo Cody mystique sure ended fast,” began a typically scathing put-down in The Hollywood Reporter. The aggregator site Rotten Tomatoes rated it just 40% “fresh” (by contrast, Juno was 93%). The film wasn’t scary enough, it wasn’t witty enough, it wasn’t sexy enough, they said.

Or maybe it just wasn’t the movie they wereled to expect by the studio’s full-on Fox-y marketing campaign, which emphasised everything of obvious interest to teenage boys and missed precisely what’s intriguing about it.

Directed by Karyn Kusama, who made Girlfight and Aeon Flux, this is a rare horror film, written and directed by women. They called it their “Trojan horse movie”, because underneath the scares and gore it sneaks in its real interest: the relationship between the good girl, Needy, and the hot girl, Jennifer. Jennifer (Fox) develops a thing for eating boys — yes, literally — after a nasty run-in with a satanic indie band. The only person who knows the truth is her lifelong best friend, Needy (Mamma Mia’s Amanda Seyfried), and she doesn’t know which way to turn.

“It’s the age-old madonna-whore conundrum,” Cody acknowledges, the first time we meet, on the film’s set in Vancouver. “What kind of woman are you allowed to be — and you aren’t allowed to be both. But the truth is, no girl is a Needy and no girl is a Jennifer: everybody is a little of both.”

On this occasion (she has since switched to a punky platinum-blonde hairdo), Cody sports pigtails, pink heart sunglasses, a leopard-print coat, white high heels and knee-length black socks with white stripes. The word “yes” is tattooed on her left wrist. She looks like she stepped out of a manga comic — though her black nail varnish is chipped and scuffed, and I wonder if she bites her nails.

She says she wrote the script for Jennifer’s Body in two months, in Minneapolis, shortly after selling what turned out to be her first, the Oscar-winner Juno. “I’d always wanted to write a horror movie. As a kid, they were forbidden. I’d go to my friend’s house and watch A Nightmare on Elm Street and Halloween on cable. Now if I go to the movies, 90% of the time it will be a comedy or a horror movie, because I want to have that visceral reaction in the theatre.”

The ideas spill out rapidly and apparently without self-censorship. She is as confident, positive and pop-savvy as one of her heroes, Quentin Tarantino. She rejects the idea that Jennifer’s Body isn’t really horrific, though she does acknowledge that her version of a monster movie is a little left-field. “I thought to myself, if I’m going to write a horror movie, then I want to write something that is actually going to scare me,” she burbles. “And when I tried to come up with the scariest thing I could think of, it wasn’t a vampire, a werewolf or any kind of bogeyman. I thought about a girl I knew in high school.

Explore Film

• Film Reviews

• Trailers

• DVD

• London Film Festival

Editor's choice

• The super cool Vampire Weekend
• Streep and Baldwin get complicated
• Guy Ritchie on Sherlock Holmes