It's only paranoia if they aren't out to get you.

That's one way of summing up "Conspiracy Theory," a government paranoia thriller that benefits greatly from a pair of sterling star performances (Mel Gibson and Julia Roberts) and liberal doses of humor.

Gibson plays a genuine, out-of-his-mind, talk-a-mile-a-minute Manhattan cabbie who publishes a newsletter about government conspiracies he gleans from the headlines of daily newspapers. And though he only has five subscribers, his publication touches a nerve with somebody and he finds himself on the run from mysterious bad guys.

Roberts, a Justice Department lawyer whose father was recently murdered, is his only ally. She's having a hard time adjusting to the loss and is uncertain what exactly happened to her father. And somehow, Gibson feels a part of her situation.

Early scenes in the film play up the comedy, with Gibson driving around town regaling his passengers with off-the-wall theories. "The entire Vietnam War was based on a bet that Howard Hughes lost to Aristotle Onassis," he says, rattling off information like an even more hyper Robin Williams.

In fact, this character seems like something tailor-made for Williams. He's manic, loaded with zany ideas, spits out rapid-fire one-liners like a verbal machine gun. . . .

But Gibson easily makes it his own, his eyes darting about, his face contorted to express self-doubt and his voice hesitating as he delivers the sometime hysterical dialogue. The latter is especially tricky, whether he's talking to himself or others as he goes on about a unique assassination attempt that will employ an earthquake in Turkey as the U.S. president visits. Then there's his assertion that political assassins must have three names — James Earl Ray, Lee Harvey Oswald . . . but is it Sirhan Sirhan or Sirhan Sirhan Sirhan? And he lives in an apartment that would make Oliver Stone proud, with locks on the door, locks on the refrigerator — even locks on the coffee!

This kind of nonsexy role is a risky step for a star of Gibson's magnitude. The character loses sympathy for a time late in the film, and there is always some question as to whether he's a part of the evildoings, or whether he's actually insane and capable of unexpected violence. But Gibson pulls it off with style. He's funny, he's scary and he even manages to subvert some of his natural screen charisma. But only some of it.