The exuberant new comedy "My Favorite Year" owes much of its impact and originality to a superlative cast of character comedians. Richard Benjamin's obvious acumen at selecting and guiding funny performers makes his directing debut an exceptionally promising, happy event.

Also a revelatory event. Opening today at area theaters, "My Favorite Year" endears itself in a resounding way because two members of the cast, Peter O'Toole in a leading role and Lainie Kazan in a relatively brief supporting role, seem brilliantly inspired and professionally transformed. While Benjamin succeeds in shifting careers deftly behind the camera, O'Toole and Kazan achieve far more remarkable career transitions in front of the camera by demonstrating a genius for comic impersonation that could never have been suspected when they first appeared on the scene.

Indeed, I'm not sure it would be possible to recognize the Peter O'Toole and Lainie Kazan who have emerged as dazzling farceurs as the middle-aged incarnations of their former selves. One recalls O'Toole as the delicately handsome actor who became prominent embodying sensitive, suffering adventurers such as Lawrence of Arabia and Lord Jim, while Kazan began as an avidly carnal vocalist dreaded for torturing torch songs into cringing submission.

Entrusted with funny roles -- O'Toole as Alan Swann, a magnificent drunken wreck who once reigned in Hollywood as king of the swashbucklers, and Kazan as Belle Carroca, a beaming Jewish mother and hilarious improvement on Molly Goldberg -- they respond with sublimely funny performances.

"My Favorite Year" creates a warmly receptive mood from Frame One by playing Nat King Cole's recording of "Stardust" over the opening credits. Remarkably fleet and sure-footed in the early going, Benjamin promptly establishes a fresh nostalgic setting for show business farce, plunging us into the backstage turbulence of a live, weekly television comedy series originating in New York City in 1954.

Our guide and nominal hero is a cherubic young gag writer, Benjy Stone, nee Steinberg, played by newcomer Mark Linn-Baker, who suggests a Cupid adopted and tamed by tenderhearted foster parents in Brooklyn. In fact, Kazan plays his mother, who resides in Brooklyn with her second husband, a former boxer, Benjy's dad having "passed away and eventually died," as Belle fondly reveals in one inimitable speech.

Benjy is the junior member of a small, awesomely incompatible writing staff whose senior members, Bill Macy as the bossy, irritable Sy Benson and Basil Hoffman as the bland but defiantly mute Herb Lee, are not on speaking terms. Some ancient grievance has evidently provoked Herb into giving Sy the silent treatment -- he transmits all his comments through Alice (Anne De Salvo, who played the hooker picked up by Dudley Moore in "Arthur"), their secretary, who gets a special sneaky charge out of broadcasting Herb's whispered put-downs of Sy.

Benjy and his exasperating mentors are employed to write funny stuff for a curt, brooding, strangely preoccupied benevolent despot named Stan "King" Kaiser, the star of "Comedy Cavalcade." Joe Bologna's portrayal of Kaiser is evidently meant to evoke the rather intimidating and perplexing image of Sid Caesar that emerges from memoirs of the heyday of "Your Show of Shows." While it's an eerily accurate impersonation, it may be too accurate to seem amusing. Kaiser's sheer dominance of the operation is comprehensible. I'm not sure, though, that Benjamin and the writers, Norman Steinberg and Dennis Palumbo, presumably building on the recollections of "Show of Shows" alumnus Mel Brooks (whose production company developed "My Favorite Year"), succeed in depicting how such a powerful but forbidding personality is transformed into an ingratiating, uninhibited clown before the television camera.

As preparations begin for the next show, Benjy volunteers for the treacherous assignment of staff chaperone to the illustrious but alcoholic Swann, the scheduled guest star. In his first meeting with Kaiser, Swann falls in a drunken heap. (Among other expert farcical skills, O'Toole has become wonderfully versatile at making his body go "tiiiimmberrrr.") Kaiser's gut reaction is to get rid of this inert celebrity, but Benjy persuades him to reconsider and agrees to watch out for Swann. Grateful for the reprieve, the actor responds with sincere gallantry to Benjy's act of kindness and hero-worshiping admiration. Over the course of a mischievous, crisis-strewn week in each other's company, Benjy and Swann develop a touching and mutually beneficial camaraderie.

The movie has its share of iffy or disappointing elements. For example, I'm not sure that Linn-Baker's cuddly, chipmunky charms can be tolerated in large doses, and I can't understand why Jessica Harper's talents are squandered on the token role of Benjy's beloved, a production assistant called K.C.

The material also tends to sputter in the stretch, but it generates so much good will and sustains such hilarious momentum for the first 60 or 70 minutes that the slack has relatively little time to accumulate. More often than not, the performers are so adept that they function as fail-safe devices for the half-baked or never-so-hot situations dreamed up by the writers. For example, a rather belabored slapstick episode in which Swann is determined to crash a party by lowering himself onto a terrace from the roof of an apartment house is saved by the look of benign nonchalance on O'Toole's face as he dangles over the brink. Likewise, the dinner engagement in Brooklyn that brings Swann into the bosom of Benjy's family is a groaner of a situation redeemed by the savory daffiness of the actors.

Lou Jacobi as Benjy's uncle, Cameron Mitchell as a goonish union boss and Selma Diamond as a long-suffering, gravel-throated wardrobe mistress also distinguish themselves within the ensemble. Diamond is entrusted with the most loaded straight line in the script, "This is for ladies only," addressed to O'Toole when he blunders into the wrong lavatory at the television studio. While it shouldn't be repeated, his cheerfully literal-minded retort is bound to be long remembered.