CAPTION: Picture, Burt Reynolds in "Rough Cut" "Rough Cut" is all too aptly titled. Despite Burt Reynolds, cast as a peerless jewel thief, and Lesley-Anne Down, the beautiful bait used to entrap him by a rather desperate Scotland Yard inspector played by David Niven, "Rough Cut" lacks the polish necessary to become the movie you fervently wish it could become an elegant, witty, erotically insinuating romance about glamorous thieves engaged in clever cat-and-mouse capers. The principal flaws are Don Siegel's stiff-jointed, direction and a lackluster plot -- in which Niven attempts to snare the elusive Reynolds before Niven is forced to retire. But the maneuvering leads to a climactic big caper that is made to appear both tedious and bewildering. Nonetheless, "Rough Cut" is not without a certain sparkle. Reynolds and Down establish amusing contact instantly, exchanging quick suggestive glances and flickering smiles during the opening credits, set at a fancy-dress party in a London mansion. Their attractiveness and the playful eye contact promise sexy, lighthearted diversion. But while the performers sustain this agreeable rapport, the material tends to muffle rather than refine the erotic signals. A crude element in the script also reveals itself instantly. Reynolds is given an ardent, inebriated companion, played by Susan Littler, who must be ditched, after some idle foreplay, before he can renew mating games with Down. Littler's character is a superfluous prop, but the movie shows a persistent weakness for such props. The ugliest -- an episode that deserves to put the rather indulgent PG rating in jeopardy -- occurs during a Continental business trip when Reynolds, planning a multimillion-dollar diamond robbery, is recruiting accomplices. One prospect, the manager of an Amsterdam brothel, is interviewed and rejected only to introduce a hint of downright nasty vice and allow the hero to give him a thrashing. This detour into gratuitous rough stuff violates the romantic, sophiscated tone the material needs to sustain and perfect. It makes no sense to present Burt Reynolds and Lesley-Anne Down as idealized sex objects and then send them on disillusioning fool's errands into red-light districts. Producers may crave such side trips far more than popular audiences do. "All I ever wanted to be was Cary Grant," Reynolds once told an interviewer. In "Rough Cut" he first tries to ingratiate himself with the heroine by doing a Cary Grant impression, and it is ingratiating, although the script insists that Down haughtily put it down. It's apparent that playing a role in the Grant style appealed to Reynolds and a pity that "Rough Cut" is too tarnished to rise to the occasion. It's possible that this faulty vehicle will test Reynolds' own appeal in a way that accomplished entertainments like "Starting Over" and "Smokey and the Bandit" wouldn't. "Rough Cut" may depend on its star's standing with the public. The material doesn't justify attendance on its own merits, but the presence of a winning stellar personality could compensate for the weakness. Reynolds' verbal timing and command of the fleeting humorous expression are frequently as delightful here as they were in vehicles that didn't work at cross purposes with him. Nevertheless, there's a somewhat strained undercurrent in his prevailing good humor and charm. In too many scenes one feels that the dialogue has unaccountably faltered, obliging the star to resort once too often to his reassuring smile or cheerful cackle. The agreeable facade doesn't shatter, but Reynolds skates along very thin ice over a vaguely slimy pond. Lesley-Anne Down often looks as ravishing as Ava Gardner in her prime. The heady sensation crated when her generous lips expand into a brilliant smile is an incidental pleasure well worth overindulging, but it would be even better if she had a role that permitted her more relaxation and confidence (and a costumer and hairdresser who consistently enhanced her beauty). The heroine's dialogue seems to have been contrived under the influence of vague recollections of Grace Kelly in "To Catch a Thief." The role affects a hauteur that doesn't become Lesley-Anne Down somehow. It also appears to tax her acting technique, which isn't nearly as sleek as Kelly's. I suspect that the very idea of presenting her as a bitchy-elegant society girl inhibits Down rather more than it flatters her. There are moments when she seems to stiffen up as severely as Kim Novak at her least secure. Whoever dressed Down (what a punprone last name!) must have been urged to remember Audrey Hepburn in "Charade." Instead of imposing vaguely similar hats and gowns on Down, whose physique appears to cry out for a different style of streamliming, it might have been more helpful to hire the director of "Charade," Stanley Donen. I can't recall that Don Siegel, best known for "Dirty Harry" and the original "Invasion of the Body Snatchers," and fresh (in a manner of speaking) from "Escape from Alcatraz," has ever displayed an aptitude for frothy or dazzling escapism. He doesn't show any unexpected sprightliness in "Rough Cut" either. As disappointments go this season, "Rough Cut" is relatively tolerable. Reynolds, Down and Niven are more amusing to watch than most of the available attractions, and you keep hoping for glamorous pleasure as this picture wends its fitful way. For those in an undemanding mood, it may even appear to satisfy the craving it merely stimulates.