Star Turns, in Theory : Mel Gibson and Julia Roberts try roles that go against their usual.

Even a blind hog, or so they say, finds a truffle now and then. So could it be that Jerry Fletcher, conspiracy theorist extraordinaire, has actually stumbled on the truth?

Fletcher is a New York City cabdriver who unnerves his passengers and everyone else in earshot with the crackpot ideas that go into his newsletter, circulation all of five. He believes that fluoride weakens your will, that the Grateful Dead tour so much because they’re in the British Secret Service, that the Vietnam War was fought over a bet Howard Hughes lost to Aristotle Onassis.

It all sounds farfetched, but some very powerful people take Jerry Fletcher seriously enough to want to kill him. The problem is, he can’t figure out over what. Only Alice Sutton, the plucky Justice Department lawyer he’s developed an awkward crush on, even half believes him, and it’s not clear what these two can do against the bogeymen of the world.

That, in brief, is the outline of “Conspiracy Theory,” written by Brian Helgeland, and as premises go it is a promising one. It might even have made a good film, but it hasn’t. In the hands of stars in denial about their stardom and a director who can’t be bothered to take things seriously, it has come out implausible and unsatisfying, a comic thriller that is not especially funny or thrilling.

The stars are Mel Gibson and Julia Roberts, and while you might think being these two on screen sounds pleasant enough, Gibson and Roberts beg to disagree. They use this film as a kind of joint plot to avoid the kinds of parts that made their careers, a hooky-playing opportunity to escape from what they’re good at. Go figure.

Gibson, best known for being handsome and heroic, is frankly unconvincing as the mentally defective, just about drooling conspiracy theorist who continually mutters to himself and secures his refrigerator with a combination lock.

Eager though he is for a chance to play offbeat, to hide his classic features behind a coating of oatmeal (don’t ask), Gibson can’t camouflage the fact that he is miscast, that the part needs more of a traditional character actor to be effective. But a character actor wouldn’t make a box-office-secure romantic pair with Roberts, so plausibility soon went out the window.

As for Roberts, her determination to play as many dour and troubled characters as possible is becoming the stuff of show business legend. As attorney Sutton, she gets to be upset about the recent mysterious death of her father as well as the constant attention of the crazed Fletcher, and even manages a scene where she breaks down in agony with her hands covered in blood.

While Roberts, like Gibson, has the perfect right to choose whatever parts she pleases (and she chose exceptionally well in “My Best Friend’s Wedding”), the truth is that in roles like this, where the actress rations her smile as parsimoniously as Ebenezer Scrooge, she is simply not as convincing or involving a performer as roles where it’s in the nature of her character to laugh freely. No one ever said life was fair, even for actors.

The evil force in this morality play is Dr. Jonas, a psychiatrist with the highest governmental connections, sharply played by Patrick Stewart, last seen in “Star Trek: First Contact.” When Jonas comes looking for Fletcher, the plot kicks into its anemic version of high gear, bringing the present and the past together in a way that ought to be more involving than it is.

Riding herd on all of this is veteran director Richard Donner, glimpsed in an unbilled cameo as Fletcher’s first worried taxi passenger. With theatrical credits that go back more than 30 years and major hits like the “Lethal Weapon” series behind him, Donner is confident enough of mass appeal to allow his films--and this one is no exception--to slip into a genial contentment with the obvious.

So it’s Donner we have to thank for indulging Gibson’s mugging when he should have been reining it in. And because the director tends to look on everything as some kind of a gag, the film’s exposition is listless and the tentative feelings between Fletcher and Simmons are unconvincing. A double star vehicle where neither party is eager to step up and be the star, “Conspiracy Theory” is a mystery that isn’t worth the effort to solve.

* MPAA rating: R for some violence. Times guideline: a graphic scene of torture.

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‘Conspiracy Theory’

Mel Gibson: Jerry Fletcher

Julia Roberts: Alice Sutton

Patrick Stewart: Dr. Jonas

Cylk Cozart: Agent Lowry

A Silver Pictures production in association with Shuler Donner/Donner productions, released by Warner Bros. Director Richard Donner. Producers Joel Silver, Richard Donner. Executive producer Jim Van Wyck. Screenplay Brian Helgeland. Cinematographer John Schwartzman. Editor Frank J. Urioste. Music Carter Burwell. Production design Paul Sylbert. Running time: 2 hours, 15 minutes.

* In general release throughout Southern California.