Quick, somebody give Kevin Spacey a bad guy role before he becomes as bland as Harrison Ford!

Just two years ago Spacey the Great was at the apex of his craft, unwrapping layers and layers of psychological subterfuge in "American Beauty" as a fettered suburban father having a volcanic eruption of a mid-life crisis. The man could project volumes of personality and complexity with the subtlest of glances and garner empathy even for his characters' ignominious acts.

But recently Spacey has stopped stretching as an actor, taking tepid, compassionate roles that come to rely on trademark tics he's developed. That curious puppy head tilt he once employed so deceptively in "The Usual Suspects" is one. His soft, lilting speech patterns that imply trustworthy gentleness is another.

He laid them on thick in "Pay It Forward," and now in the ambiguously metaphysical "K-PAX" his performance consists entirely of Spacey-isms, as he plays a dulcet John Doe committed to a Manhattan psychiatric hospital because he claims to be an alien.

He calls himself Prot, sports a distantly pacific smile, eats bananas without peeling them and impishly makes declarations like, "You humans. Sometimes it's hard to imagine how you've made it this far." It's standard-issue stranger-in-a-strange-land stuff that allows Spacey to frolic in idiosyncrasies but doesn't give him much substance to chew on, even as his shrink (Jeff Bridges) begins to doubt his diagnosis of Prot as "the most convincing delusional I've ever come across."

If Prot is a human nutcase, the psychologist wonders, why can his eyes detect ultraviolet light? Why is he completely unaffected by a daily dose of 300mg of Thorazine? He doesn't know what to believe, but he tries to uncouple Prot from his elaborate fantasy anyway, giving him a batch of questions about his corner of the galaxy -- supposedly 1,000 light years away.

The questions are drafted by a group of astronomers in Bridges' acquaintance and no layman would know the answers. But Prot does -- plus he draws a mathematically precise diagram of his solar system that solves a mystery of physics that had vexed the study of his home planet's binary star for years.

As Prot's claims gather credibility, "K-PAX" takes on a spellbinding warmth and a calculated ambiguity that counterbalance its pappy mysticism and overcooked screenplay. Prot's inexplicably therapeutic effect on the other patients in his ward is prosaic but palatable, as are the hypnosis sessions that reveal a haunted human past without definitively substantiating his humanity.

But there are times when director Iain Softley ("Wings of the Dove," "Hackers," "Backbeat") so readily indulges his desire for emotional potency that the movie dips into mawkishness and contrivance. In the last act, Bridges follows Prot's past all the way to New Mexico to have a pivotal conversation he could have just as easily had over the telephone. But Softley apparently felt the need to ply the closing scenes with the kind of highly scripted melodrama you can only get from superfluous story bombshells.

Playing out as it does, "K-PAX" bears an atmospheric and characteristic resemblance to 1984's "Starman," in which Bridges himself played a quizzical alien in human guise. Both films ultimately overcome self-consciously mannered lead performances and heavy-handed sentimentality to emerge as gratifying, if obtuse, works of obliging machination.