(CNN) -- You don't need to be a teenage boy to realize how scary girls can be. Girls know it too.
Megan Fox plays a demonic cheerleader in "Jennifer's Body," written by Diablo "Juno" Cody.
Certainly Diablo Cody does.
The Academy Award-winning "Juno" scribe proves as much in her second screenplay, a horror comedy about BFFs Jennifer (Megan Fox) and Needy (Amanda Seyfried).
These bosom buddies have a relationship that stretches back to the sandbox. They're inseparable opposites: Jennifer is a babe, head cheerleader and a bit of a snot. Needy is nerdy, conventional and a soft touch.
Except that when we first meet her, she's locked up in the local mental hospital, kicking back at the guards and muttering darkly about the murders that put her here.
Looking back, she dates this disastrous turn of events to the day Jennifer talked her into seeing an up-and-coming indie band at the roadhouse. The gig ends in a deadly inferno that claims the lives of several schoolmates, then Jennifer drives off with the band for an al fresco after-party involving satanic rites and virgin sacrifice. She survives on a technicality, but shows up later that night at Needy's place covered in blood, barfing blue goo and craving fresh flesh.
In the days after the tragedy the entire school seems to be suffering from post-traumatic stress. Only Jennifer seems unfazed by the community's second sucker punch: the gruesome fate of the star quarterback in the woods.
What's a girl to do if her best friend turns into a heartless boy-eating demon? Needy cleans up the mess and hopes it's just a phase.
The last time a horror flick tried for a distinctly female point of view the result was "Twilight," which was more of a wan gothic romance than a chiller. Directed by Karyn Kusama, who made "Girlfight" and "Aeon Flux," "Jennifer's Body" isn't anything like that. It's a much edgier, snappier, bloodier film, aimed primarily at young hipsters and horror fans -- plus anyone else curious to see Megan Fox get nasty. (That's got to include a sizable chunk of the audience, surely?)
So smoldering hot that at one point she coolly sets fire to her tongue, Fox makes a convincing vixen, callously picking up victims whenever her luster begins to fade. It's not hard to imagine she can have anyone who takes her fancy -- even Needy is not immune to her charms.
Ironically, though, Jennifer is really the needy one here, while her friend eventually finds the self-possession to stand up to her. Similarly, you might come for Fox, but it's Amanda Seyfried's expressive range and toughness that leaves a more lasting impression. The "Mamma Mia" starlet is destined for good things. iReport.com: Share your review of "Jennifer's Body"
The bitingly smart, funny teen-speak is carried over from "Juno," along with sharp pop culture references and a sassy feminist attitude, but the million-dollar question has to be: Is it scary?
Only occasionally, I'm afraid.
The gore scenes come with weird little flourishes: Jennifer's mounting body count attracts a crowd of curious woodland creatures, sounding a sweetly sick echo of "Bambi," and a climactic showdown is set in a spectacularly fetid abandoned swimming pool (don't they drain those things?). But the jokiness does tend to undercut the terror.
I suspect the film will prove too freaky for "Juno" boosters, and could have used more straight-ahead scares and sustained suspense to appease the hardcore horror geeks.
That said, this entertainingly oddball offering does twist fresh kinks into a genre that's always crying out for new blood.
"Jennifer's Body" is rated R and runs 102 minutes.
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