Jim Carrey entertains himself mightily in Liar Liar, and his enthusiasm is infectious. Dressed in a bronze-gray double-breasted suit that suavely complements his handsomeness, he’s playing a ”real person” this time — gone is the Li’l Abner pompadour of the Ace Ventura films, the outsize lisp of The Cable Guy — and part of the fun of the movie is watching his madness erupt from an image of dapper, clean-cut normality. Carrey’s Fletcher Reede is a defense attorney who reflexively tells people what they want to hear. Skidding through his day on an oil slick of euphemism, he hustles clients and judges, his ex-wife and coworkers, even himself. He’s like an addict in denial; he barely recognizes that he’s a pathological liar. All of this changes when his moptopped son, Max (Justin Cooper), having suffered one too many broken promises, makes a birthday wish that Dad stop lying for 24 hours. The wish comes true. Suddenly, Fletcher can’t lie — he becomes physically incapable of spitting out anything but the truth, no matter how rude, embarrassing, or destructive. He becomes a pathological id.

It would be easy to imagine this premise used as a springboard for social satire, a parody of the way that all of us, to a greater or lesser degree, have to varnish the truth in order to get through the day. Overnight, Fletcher becomes a jack-in-the-box of sincerity. Barreling through his law office, he starts firing off insults at everyone in sight — the assistant with the hideous geek-punk braids he’d complimented only the day before, the schnook with a giant zit at the end of his nose. Later, he does the same thing at a board meeting, and the vicious quips pop like firecrackers.

Carrey, in a sense, has always been a compulsive liar. His comic persona is rooted in sarcasm, which he stylizes to such a manic degree that it becomes a mock form of conviction. (He’s utterly sincere about his insincerity.) In Liar Liar, honesty turns him into a surreal comic hellion. The truth is like a tornado that keeps erupting out of his mouth and rebounding through his body. Desperate to see if he can still lie, Fletcher clutches a blue felt-tip pen and works himself into a convulsive frenzy trying to utter the sentence ”The color of the pen is red.” He works to get his lips around it, to write it on a piece of paper, but he’s like a man in a straitjacket. The more he tries to lie, the harder the truth straps him in. Carrey, tying his face in knots, builds a demented rhythm out of suppression and release, so that when the truth bursts out of him we feel his simultaneous horror and joy. Fletcher knows he’s ruining his life, but he’s like someone who has to regurgitate and, finally, does.

Liar Liar was directed by Tom Shadyac, who made Ace Ventura: Pet Detective and showed a surprising human touch in the Eddie Murphy remake of The Nutty Professor. Shadyac is trying for a human touch here, too, but this time the emotions feel processed. The sentimental story, in which Fletcher the fickle dad learns to be true to his son, is straight out of a Chevy Chase movie or Jingle All the Way. Then again, Liar Liar isn’t a ”family” comedy or even a real satire. Its central relationship isn’t the one between Fletcher and the people he now has to tell the truth to. It’s the one between Jim Carrey and himself.

I wish I could say that Liar Liar was one long Jim Carrey high. Like most of his films, it’s at once witty and repetitive, inspired and exhausting. In court, Fletcher has to represent a wealthy adultress (Jennifer Tilly) in a divorce case. The situation of an unscrupulous attorney denied his bag of tricks is almost too obvious, and by the time that Fletcher, insane with frustration, walks into the men’s room and literally beats himself up, the film has begun to exhibit some of the raucous single-mindedness that undermined The Cable Guy. Carrey is undeniably a virtuoso. At his best in Liar Liar, he’s like the missing Marx Brother — Sleazo. At this point, though, a little less of him might be more. He could make an even funnier movie if he started to let some other performers bounce a few good lines off him.