At the beginning of “Yes Man,” Carl Allen is a grouch, a curmudgeon, a wet blanket. His relentless negativity — his job as a loan officer provides him with plenty of opportunities to say no — is less a matter of temperament than of circumstance: apparently Carl never recovered from the breakup of his marriage. But then a visit to a self-help seminar led by a guru (Terence Stamp) who preaches the power of yes transforms Carl into a wild, unpredictable fellow, a giddy, spontaneous goofball, a gangling, motormouthed, rubber-faced id. In short, Carl turns into Jim Carrey.

But “Yes Man,” dutifully directed by Peyton Reed (“Bring It On,” “Down With Love”) is too sluggish and slapdash a vehicle for its star’s prodigious wackiness. The movie’s premise is not far from that of “Liar Liar,” one of Mr. Carrey’s biggest hits of the mid-’90s, in which he played a chronic dissembler suddenly compelled to tell the truth. That film was a philosophical tour de force compared with this one, which arrives at the startling conclusion that while saying no to everything is bad, saying yes to everything is not ideal either.

Most of us learned at least a rudimentary version of this lesson around the age of 5, and one of the many problems with “Yes Man” is that it is insufficiently infantile. Mr. Carrey looks too grown up, too tired to plunge into the reckless silliness that the story demands, and the story doesn’t really demand enough of it. Once Carl has banished no from his vocabulary, what does he say yes to? Flying lessons. A class in conversational Korean. Every loan application that crosses his desk.

That last bit has a somewhat unpleasant, if inadvertent, topical resonance, but never mind that. Some of the things Carl does are a bit wilder than learning to play the guitar. He accepts a sexual proposition from an elderly neighbor, which leads to a gag that was already tired when it popped up in “Soul Men” a few months ago. He wraps his head in cellophane tape. He goes on a Red Bull binge. He provokes a fight in a bar after snorting some Tabasco sauce.

And he falls in love with a pale-eyed, mildly eccentric cutie named Allison, played by Zooey Deschanel, who took on similar waif duties opposite Will Ferrell in “Elf.” The role of Mr. Carrey’s romantic counterpart is never an easy one, given his manic energy and the childish narcissism that is the basis of his shtick. Only Kate Winslet, engaging with a more subdued incarnation of Mr. Carrey in “Eternal Sunshine of the Spotless Mind,” has been able to distract him from himself. Ms. Deschanel never really has a chance, and instead functions as a kind of sidekick, breezing through some semi-funny set pieces on the way to an obligatory smiley-teary ending.

Mr. Carrey has some good moments, as do a few of the supporting performers, notably Bradley Cooper as Carl’s best friend and Rhys Darby as his socially maladroit boss. But “Yes Man” rarely rises to genuine hilarity. It takes no risks, finds no inspiration and settles, like its hero, into a dull, noncommittal middle ground. Should you see this movie? Maybe. Whatever. I don’t care.

“Yes Man” is rated PG-13 (Parents strongly cautioned). It has some swearing, sexual references and brief nudity.