It is to be expected that "The Roommate" is totally and utterly formulaic, borrowing elements from every past movie that is almost exactly like this one. 1987's "Fatal Attraction." 1992's "Single White Female." 1993's "The Crush." 2002's "Swimfan." 2009's "Obsessed." There's more—many more—where those come from, each one a variation on the same premise wherein a mentally unhinged woman first becomes possessive of someone else, then outright dangerous. If she can't have him or her, no one can. "The Roommate" delivers precisely what is expected up to a point, albeit in the most rudimentary of fashion, before resorting to an ending that is aloof and irresponsible even for the genre.

University of Los Angeles freshman Sara Matthews (Minka Kelly) has no sooner moved into her dorm and gotten wasted off spiked punch at her first frat party when she meets new roommate Rebecca (Leighton Meester). The two of them hit it off fairly well, bonding over their love of coffee shops and art, until Sara slowly starts to come around and comprehend the warning signals flashing in her face. Rebecca, you see, isn't just a little quirky, but a borderline schizophrenic who has opted to stop taking her anti-psychotic medication. Pretty soon, no one who attempts to get close to Sara is safe—not gal pal Tracy (Aly Michalka), not new boyfriend Stephen (Cam Gigandet), not ex-boyfriend Jason (Matt Lanter), not lecherous art professor Mr. Roberts (Billy Zane), and not artist aunt Irene (Danneel Harris).

It is customary in these kinds of thrillers—a subgenre frequently described as "...From Hell" movies—for the eventual villain to at least start the picture off under a guise of normalcy before their loonier side is unleashed. In "The Roommate," the viewer need only take one glance at Rebecca to know she's as crazy as a crap-house rat. From there, Danish director Christian E. Christiansen (making his U.S. directing debut) and first-time screenwriter Sonny Mallhi seemingly check off their list of horror tropes one at a time with no attempt to put a spin on them. There's a shower scene built to make one tense, then jump (it does neither). Rebecca steals a cherished necklace that belonged to Sara's dead sister. There are, of course, the threats, the blackmail, and eventually much worse directed at Sara's acquaintances. Suffice it to say, when Cuddles the kitten enters the equation, it's a done deal that it won't be around by the start of the third act.

While much of this is going on, heroine Sara—depicted as virtuous in the way that she remains responsible even after getting plastered—is unbelievably slow to catch on. Indeed, it's not until they visit Rebecca's home and her mom (Frances Fisher) comes right out and asks Sara if her daughter has been taking her medication while away at school that Sara fully grasps the seriousness of her roomie's erratic behavior. Minka Kelly (2009's "(500) Days of Summer"), formerly of TV's superb "Friday Night Lights," is the sort of actress one can just look at and immediately know she's got a brain behind her attractive physical appearance. When forced to dumb things down, as she is here, Kelly has trouble selling it and looks put out by the script's deficiencies. As Rebecca, Leighton Meester (2010's "Country Strong") lays her conniving villainy on thick, but is reasonably affecting before the film makes an outright mockery of her very serious psychological problems. The undercurrent of suggested lesbianism on Rebecca's part should be credited for even daring to go down such an avenue, but this, too, is used as more of a gimmick and makes the viewer long for 2010's far more intelligent "Chloe," directed by Atom Egoyan. The other actors only have the chance to be serviceable, with Aly Michalka (2010's "Easy A"), Cam Gigandet (2010's "Burlesque"), Danneel Harris (2010's "The Back-Up Plan"), and Billy Zane (2006's "BloodRayne") on hand to be thinly drawn targets of Rebecca's wrath.

Viewed on a purely entertainment-based level, "The Roommate" is diverting in a highly inconsequential way, a watchable 92 minutes of mediocrity. One can guess what is going to happen from scene to scene up until the shockingly heartless, falsely happy ending. Without giving away the particulars, the conclusion simultaneously does a disservice to both lead characters. For Rebecca, she is robbed of the last shreds of her humanity and her mental illness is shoved into the background in lieu of a stock fight-to-the-finish cat-fight. For Sara, she ceases being sympathetic with her flippant attitude toward actions that, were they real, would likely alter her life for years to come. As the film would have it, however, Sara doesn't give a second thought or care about what she has done. The final image, intended as something of an amusing punchline on all that has just occurred, simply comes off as mean-spirited. "The Roommate" is just about as inspired as its title.