CLAUDE EASTMAN (Dudley Moore) looks like an unhappy man. "In Italy," his wife Daniella (Nastassja Kinski) purrs, "we show our emotions." "In New York," Claude replies, "we just sulk." But ultimately Claude's sulking gives way to delusions of honor regained. Weeks before. Claude was the happiest man around: a successful orchestra conductor and top celebrity, infatuated with his dazzlingly beautiful young wife. "I love her," he says at the beginning of the movie, adding, "but I am going to kill her." What went wrong is the premise for the plot of his latest movie. Unfaithfully Yours.

Back from a lengthy concert tour, Claude meets his agent Norman Robbins (Albert Brooks) and listens to some gibberish about a private detective's report on his wife. Confused about the detective. Claude denies any jealous tendencies. His manservant, Giuseppe (Richard Libertini), misunderstood Claude's request "Keep an eye on her" for the Italian equivalent of "Get a private eye to follow her." With the requisite veneer of trust, Claude poo-poos the report and falls back into the arms of Daniella.

In due course, Claude predictably ends up in the sleazy offices of the private detective and falls for "evidence" of his wife's infidelity. Once deserted by his own younger wife, the detective shows Claude videotapes showing some amorphous Argyle sock clad man leaving his luxurious apartment late at night. Still retaining his facade of trust. Claude suggests the man could be a "repairman." At 1.00 in the morning? "The only thing that breaks at that time of the morning," the detective says, "are the hearts of men like us." Fitting every scrap of evidence into his now increasingly suspicious mind, Claude concludes the socks of his wife's lover belong to Max Stein (Armand Assante), his protege and rising violin player. Still in New York, Claude is ready for more than sulking.

In Unfaithfully Yours, Dudley Moore delivers his best comic performance to date, confirming that his bubbling talents in Arthur can sparkle even when his character doesn't guzzle champagne. Instead of playing the caricatured drunk (Arthur) or the caricatured sex conquistador (10), with Claude, Moore gives his most balanced characterization yet. Moore, an Oxford graduate degree holder in music and an accompolished jazz artist, plays the conductor comfortably and convincingly.

As Claude, Moore engages rival Stein in a spectacular "dueling violins" scene and sends the apprentice crashing into the tables. With equal case, Moore brings freshness and warmth to a potentially hackneyed older-jealous-husband-suspects-young-sexy-wife routine.

ALTHOUGH ORIGINALLY CAST with the late Peter Sellers in the lead, Unfaithfully Yours works largely because of Moore's tremendous comic talent. Without resorting to the clownish absurdity of a Steve Martin or the profanity punctuated anger of a Richard Pryor. Moore delivers a comic performance rich in warmth and charisma. Even at slapstick, Moore delivers, pathetically catching his foot in a wall that he kicks in. In some jokes, you just have to be there, in Unfaithfully Yours, just Dudley Moore has to be there.

Several excellent supporting roles complete the comic balance. As his agent, Norman, Albert Brooks plays the perfect straight man for the dynamic Moore. In a riotous scene, a freshly released from jail Claude attacks Brooks in the police station. "Police, Police." Norman cries with blue suits literally everywhere. Richard Libertini as the manservant plays the clown character, forcing Moore to focus on his more diverse and definitely more interesting talents.

Kinski, after several promising performances in Tess and Cat People, gives little here as Claude's sultry wife. Trying to look angry at Claude, she seems only catatonic. Producers Marvin Worth and Joe Wizan perhaps cast Kinski for other attributes which she displays generously and frequently. Expect no eroticism, though: the onetime star of Cat People and snake-loving poster pinup gets no kinkier than seducing Moore with a pig's mask on.

The object of all pre-pubescent female desire in Little Darlings and Eurostud in Private Benjamin, Armand Assante competently plays the lover apparent and star violinist. Where violinist Stein should be unconscious, Armand Assante keeps his eyes closed. Throughout the entire film, he plays the self-centered seducer, and yet he never takes himself so seriously as to make his character one-dimensional and boring.

Screenwriters Valerie Curtin, Barry Levinson, and Robert Klane rightfully reserve the best lines for Moore and sidekick Brooks. Though more than a series of one-liners, Unfaithfully Yours includes enough of those to engrave a few gems upon the memory.

Director Howard Zieff paces the film well enough to avoid dull moments between situations. "In comedy," Zieff says, "You are recruiting some body who already has the talent and the facility of delivering. All you, as a director, have to know is what you want and how to express it...." In this case, Zieff gives Moore enough room to reveal a wide range of comic talents without turning the movie into an extended stand-up routine.

Like many things, including certain lectures, Moore's performances seemingly run in groups of three. After an initial series of successes in Foul Play, 10, and his masterpiece, Arthur, Moore fizzled in Six Weeks, Lovesick and Romantic Comedy. (A biblical satire, Wholly Moses, remains in a class, or lack thereof, all by itself) with any luck, then, Unfaithfully Yours marks the start of a new series of successes. Currently shooting Best Defense with Supercomic Eddie Murphy. Moore has given at least one reason here to expect still more laughs.