It would be such a joy to bend the truth and say that "The Invention of Lying" lives up to the potential of its inspired premise.

The conceit - that an alternate universe exists where everyone tells the truth all the time - sets up an uproarious beginning. People say what they really think to friends, co-workers, even strangers - but rather than bringing out the best of humanity, it reveals the cruelly judgmental worst. A woman says to a man on a first date, "I don't find you attractive." The waiter admits upon approaching their table, "I'm very embarrassed I work here," then takes their drink order.

Clever stuff. But then the movie plummets precipitously from there.

It's not just the high-concept gag wears thin, which it does. The bigger problem is that Ricky Gervais, in his directorial debut (alongside co-director and co-writer Matthew Robinson), zig-zags awkwardly between dark humour and heavy melodrama. One character is suicidal and another is on the verge of dying, both of which are played awkwardly for laughs.

It certainly doesn't help that "The Invention of Lying" is lighted so hideously, everyone looks like death - even Rob Lowe and Tina Fey. This is especially obvious given Gervais' fondness for cutting back and forth between close-ups of his actors, which he does with distracting frequency.

On camera himself, he's likable enough as Mark Bellison, a wisecracking sad sack of a screenwriter who discovers the unheard-of notion of lying one day and explores its many benefits. But sharing scenes with him are Philip Seymour Hoffman, Edward Norton and Jason Bateman, who go to waste in barely-there cameos.

Gervais, meanwhile, remains in the same dryly self-deprecating mode we've come to know and enjoy, but a little bit of him - on TV's "The Office" or "Extras," for example - goes a long way. Here, his character stumbles across the fun of fibbing, tries it recreationally among friends, exploits it for accolades at work, then becomes a full-blown worldwide sensation when he makes the mistake of saying he knows what happens after you die. Basically, he makes it all up as he goes along. But suddenly, the conceited but guileless Anna (a perky Jennifer Garner), who never gave him the time of day before, finds him fascinating and wants to be with him constantly. That's a plus, too.

Gervais deserves credit for approaching the idea that God and heaven are part of an elaborate lie meant to assuage the masses - a bold move for a big-studio comedy with lots of stars - but then he backs off, as if he and Robinson hadn't thought it through all the way.

He does take time for shamelessly in-your-face product placement, though. Mark delivers his Ten Commandments-style sermon using Pizza Hut boxes, and Budweiser is consumed about a half-dozen times. So while Gervais is making fun of widely held beliefs and customs, he's also buying into them. And selling them.