"Once in Paris," now at the K-B MacArthur, is one of those marginally interesting movies - "Saint Jack" is another current example - which might justify an investment if you've seen everything else halfway decent and find pictures with unrealized possibilities intriguing.

Written and directed by Frand D. Gilroy, "Once in Paris" seems to be onto somthing fresh and entertaining, but Gilroy ins't quite the film-maker to nurture it into a satisfying experience.

Wayne Rogers stars as a screenwriter who arrives in Paris for the first time in order to do a quick reqrite on a sick script that a producer friend needs to get into production in a matter of weeks. Preoccupied with the job, he's prepared for all work and noplay until more or less healthily corrupted by Jack Lenoir, a pleasure-loving chauffeur hired by the movie company to drive him around town.

Under the circumstances the hero's work ethic is perceived to be a rather priggish affectation. He has sized up the script's problems neatly and anticipates no difficulty making the necessary changes. At the beginning it merely suits his vanity to act like a man who's too busy to be diverted from duty.

The absurd aspects of this posture become apparent to him when he checks into his hotel room. Lenoir is appalled that he's been stuck in a cub-byhole, but Rogers insists that it's fine, he'll be doing nothing but working anyway, etc., etc. The chauffeur, who knows his way around, brushes off his protests, calls the desk and insists on something suitable for a distinguished filmmaker. Minutes later Rogers is transferred to a spacious suite, which proves to be not only pleasant to work in but next door to a glamorous woman of mystery, Gayle Hunnicutt, with whom he enjoys a brief romantic fling.

The idea of the evolving friendship between the American, cautious to a fault, and the Frenchman, worldly to a fault, seems extremely promising, and Rogers and Lenoir have excellent rapport. Their acting is subtle enough to offset the simplistic contrasts Gilroy perceives in the different nationalities and outlooks of the characters.

You warm to the humor inherent in this situation, but Gilroy gets little comic mileage out of it. He's got distinctive characters. It's the process of character exploration that seems to break down. Once introduced to these men, you expect more than Gilroy delivers, just as you expect more of the Paris setting than he's capable of using expressively.

Roger's infatuation with Hunnicutt degenerates from romantic diversion into sentimental drag. When Gilroy not only gets misty-eyed over this brief encounter but exploits it as a violent source of conflict between Rogers and Lenoir, his judgment seems to have fled completely. You wish he'd called in a good-humored rewrite specialist to repair the damage.

It may help to know that Lenoir expert as he seems, was not a professional actor until recently. Gilroy encountered him in Paris when Lenoir was working as a chauffeur. He radiates total professional assurance. If Gilroy did too, "Once in Paris" might have been an exceptional sophisticated comedy.