It's impossible to discuss some of the hiccups in the concept of Neil LaBute's "The Shape of Things" -- a thorny, thought-provoking contemplation of the lengths people will go for love, or what they think is love -- without giving away the startling twist the film takes in its last act.

But it can be said that in adapting his own 2001 play, the writer-director didn't augment the characters and settings with the additional depth and definition necessary to flesh out a stage production for the screen.

As a film, "The Shape of Things" is set in the real world -- on a college campus where frumpy, unassertive, full-time English major and part-time museum guard Adam (Paul Rudd) comes under the lifestyle-altering influence of a sexy, puckish, wily, funky art student named Evelyn (Rachel Weisz), whose interest and affection Adam desperately clings to because he hardly believes in it himself.

This cinematic real world requires a greater authenticity than a stage production (where everything beyond the characters themselves is an abstract), but "The Shape of Things" provides none. No one ever goes to class. No one mentions homework, papers, tests or tuition. There's no sense of the characters' lives outside their service to the story.

As not-quite-sincere Evelyn embraces awestruck Adam then tightens her grip, he goes through a slow transformation that seems positive at first -- losing weight and bad habits while gaining self-confidence, a better haircut and a new wardrobe. But his changes grow increasingly dubious (she talks him into getting a nose job) as the months go by without the two lovers really becoming any closer. This raises red flags for Adam's two best friends, abrasive know-it-all Philip (Frederick Weller) and Philip's cute, compassionate, somewhat irresolute fiancée Jenny (Gretchen Mol) -- an unrequited love from Adam's past who begins to find the reformed nerd more desirable.

Soon Evelyn wants Adam to cut these friends out of his life -- a final, willing and major alteration that leads to a bewildering betrayal, which in turn invites an profound exploration of the concepts of integrity, friendship, love, being true to one's self, and even art.

Psychologically and emotionally compelling, "The Shape of Things" is weakened by the characters' seeming lack of existance beyond these few relationships -- and even those seem under-ripe at times. Hasn't Adam ever asked Evelyn about the big art project she's is working on? Hasn't he wondered if she has friends and why he's never met them?

There's a tangible awkwardness of unresolved feelings between Adam and Jenny, but when they're alone together they seem more like two people who've just met than like friends who have never gotten comfortable around each other.

Sometimes the performances feel both under-rehearsed -- which is strange since Rudd, Weisz, Mol and Weller are all original cast members from the play -- and overly staged (part of the unwieldy transition to film). When Jenny and Adam walk and talk in a park, they pause with conspicuous premeditation at a picnic table, a swing set and an ocean cliff side as if these spots were marks at stage left, stage center and stage right.

Provocative to be sure, "The Shape of Things" makes an interesting contrast to LaBute's controversial portrait of misogyny, "In the Company of Men." But it simply feels shackled to its previous incarnation. It doesn't breath like a movie or have the elasticity of a movie. It's confined and restricted by its narrow focus on just the interactions of this foursome without them interacting with anything or anyone else.

The picture lacks the cinematic sophistication LaBute has demonstrated in the past (most recently in the narratively complex "Possession"), and as a result the credibility of the characters, their lives and the world they inhabit suffers to the point of distraction.