How do you go about promoting a film opening in Britain that has had nothing but horrible reviews and lousy box office figures in the United States? That's the position Megan Fox and Diablo Cody found themselves in this weekend - Fox being the star and Cody the writer of Jennifer's Body, a horror film about a high school girl who develops a taste for eating boys.

American critics found the film not scary enough to be a good horror movie, not as funny as they expected after Cody's Oscar-winning debut with Juno, and, despite a good deal of female flesh on show - most of it belonging to Megan Fox - not sexy enough to satisfy the teenage boys who'll turn out for anything starring Fox.

In an Observer interview, Fox came up with very little positive to say about Jennifer's Body. Like her two hugely successful Transformers movies which have helped make the the hottest 23-year-old of the moment, even she admits the new film will not help answer the question: can she act?

"It's a cool movie," she said, "but I don't think anybody's going to walk away from it and go, 'Wow, she's really talented'. I don't think it will propel me to a different level. Even I don't know how much talent I have or what I'm capable of. At best, someone might say, 'She's different in this movie than she was in Transformers'."

In fact, audiences are more likely to come out wondering about the girl-on-girl sex scene with co-star Amanda Seyfried. "Clearly I can't argue that it's not gratuitous, because it is," admits Fox. "There's no music it's just silence and the sound of spit. I feel weird watching it. It was just really uncomfortable. I felt like I was witnessing something I'm not meant to see."

For Diablo Cody, the release of Jennifer's Body has been even more awkward. Expectations were enormous after she became the toast of Hollywood with Juno. But the Los Angeles Times wrote off the new film as "a sorely lacklustre scare flick" while the Hollywood Reporter commented: "Well, the Diablo Cody mystique sure ended fast".

Coincidentally, the film's opening came within days of Cody's first TV drama series, The United States of Tara, picking up an Emmy for its star, Toni Collette. "Team Tara is jubilant. Team J-Bod is suicidal," Cody tweeted. "Playing both sides is exhausting. Can't distinguish between whoops/wails on phone."

Talking to the Sunday Times, Cody, famous for having spent a year in the sex trade before making her breakthrough as a writer, explained how the post-Juno pressure had got out of hand. "I did a lot of press for Juno because I wanted to support the film - and it worked," she said. "But the repercussions for me were disturbing and continue to disturb. Believe me, if I wanted this kind of notoriety I would have been an actor. They want attention; they're wired to enjoy it. But I am a writer first and foremost. I like to be alone with my computer."

So why did she pick the attention-getting name Diablo Cody if she wanted to go unnoticed, asked the interviewer. "I named myself Diablo Cody for the purposes of writing on the internet," she responded, "so my parents wouldn't Google my name and know that I was a stripper. "Everyone on the internet uses a handle and I used this one." Because the publishers of her first book wanted her internet followers to know it was her, the name stuck. "I certainly did not name myself for self-promotion - that is such a misconception."