J.J. Abrams has admitted that the idea for “Alias” (the best show in the history of television) came late one night in the writers’ room of his first show, “Felicity,” when the staff jokingly imagined what would happen if its awkward, painfully self-aware protagonist was transported from her melodramatic college existence into the life of a kick-ass spy. The results were legendary — Abrams brought Felicity’s emotional resonance and defiant morality into a wonderful world of action, intrigue and sex.

Fresh off the success of her debut screenplay “Juno,” Diablo Cody must have had a similar thought process in developing her second film. What if “Juno” was a balls-out horror flick? Cody — clearly inspired by the violently feminine world of “Alias” — aims to craft a less cerebral, more “90210” Tarantino flick. She largely succeeds, but one is left questioning the merit of the goal in the first place.

“Jennifer’s Body” chronicles the unlikely friendship between nerdy Needy and sexy Jennifer. When Jennifer is (humorously) murdered by a struggling indie band, a demon enters her titular Body. Only Needy can see that Jennifer is different — she is now only nourished by the blood and organs of men she seduces and then eats. Needy must stop her BFF before she eats Needy’s supportive, naive Michael Cera–clone of a boyfriend.

The Juno character in this case is split between two actresses: The vulnerable relatability is played to perfection by Amanda Seyfried as Needy (you know her as the lead in “Mamma Mia”), while the bloggy vernacular and so-what attitude is embodied by the surprisingly talented yet hopelessly alien Megan Fox as Jennifer. The unlikely best friendship between the characters is fascinating — it leaves you wishing the film had been a less fantastical tale that really could explore the pair in depth.

The movie confirms that Seyfried is one of the most talented young actresses in the industry, but her face doesn’t even appear on the movie poster. The (financially underperforming) movie belongs to the foxy Megan Fox. She snarks her way through Juno-isms with spunk: To her, hot guys are “salty morsels” and dullards are “freaktarded.” The slang is absurd, yet Fox makes it believable (albeit still stupid). In the final fight scene, her comic delivery (“You even buy your murder weapons at Home Depot? God, do you have to be so butch?”) is impeccable.

Yet none of this is surprising — anyone who has seen Megan Fox in an interview can attest to her charisma and quick tongue. The problem is, through it all, it’s impossible to feel a thing for her character. She’s built a larger-than-life persona outside the Cineplex better suited for IMAX transforming robots than the melodrama of teenage angst. She appears superior, an unfair adversary for anyone, and her moments of vulnerability fall flat. In the end, her range is about as small as her ridiculous waist — she can play the hot girlfriend or queen bee, but even a cannibalistic demon with a heart is too big of a stretch.

The film makes you worry about Cody’s range as well. Her unusual voice is sharp and entertaining as ever, but shows little development from “Juno,” where it was a better fit for the genre. She clearly has an eye (and ear) for the Teenage Condition, yet it’s clear she’s too smart for dumb horror films. A screenwriter of her caliber should not be writing two-minute lesbian makeout scenes. The movie is at its best in its most tender moments, as when we watch Seyfried with her Michael Cera-esque boyfriend or see the girls trying to pick up guys at a bar.

It’s not that the film isn’t enjoyable — the movie is frightening and funny — it’s just that it could have been so much better. Every time Cody’s story begins to help the actresses transcend the genre and their own limitations, there’s another scene of Megan Fox eating the liver of an emo guy. Each time it happens, it’s hard not to feel emo yourself.