During a pivotal scene in "Arthur," a witty and adroit new romantic farce about th eliberation of an overaged Poor Little Rich Boy, Stephen Elliott glares at Dudley Moore, his prospective son-in-law, and remarks: "You seem to find humor in everything." Elliott's character, a wealthy brute named Burt Johnson, enjoys attributing his success to a streak of rutlessness. He makes it clear that he is not about to be amused by the irrepressible antic streak in Moore's Arthur Bach, an endearing upper-class Disgrace.

Burt's doting daughter Susan (Jill Eikenberry) may be permitted the luxury of regarding Arthur as a worthy reclamation porject. But Bart regards Arthur as a necessary incovenience, the means of amalgamating two family fortunes. In his book, that is no laughing matter. The drunken, good-for-nothing runt does what's expected of him Or Else.

Of course, Burt couldn't be more mistaken. "Arthur," opening today at area theaters, sustains a thin but serviceable plot on the certainty that providence will never allow such a marriage to take place. It's subverted, naturally, by a more compatible alliance. Arthur falls for a forthright, quick-witted working girl, Linda Marolla, played by Liza Minnelli, and eventually works up enough booze-fortified gumption to defy Burt, Susan and his family.

Ironically, Burt's disapproving remark serves to clarify Arthur's disreputable charm. Finding humor in everything may be Arthur's only reliable means of defense. His way of life is patently absurd: a spoiled childhood prolonged into profligate middle age. Nevertheless, Arthur's irresponsible habits are moderated by a cheerful disposition and a spontaneous sense of humor, which is often at its most infectious when the joke is on him.

Arthur, the most ingratiating role of Dudley Moore's film career, is the latest variation on an enjoyable tradtion. Arthur recalls Harold Lloyd in a vehicle like "Grandma's Boy" or Buster Keaton in a vehicle like "Steamboat Bill, Jr." There's also a curious mixture of influences from "City Lights": Arthur combines attributes of Chaplin's Little fellow and the jovial society drunk who recruits him as a companion.

With some justification, Arthur fears that he's unsuited for conventional employment and too shrimpy to challenge the authority figures in his life. Self-deprecating humor is his saving grace and promises to make "Arthur" the biggest little sleeper of the season.

Steven Gordon, the writer-director, has done a remarkable job of evoking the pixilated artificiality of vintage screwball comedies set among the loony rich -- "Easy Living." "My Man Godfrey," "The Palm Beach Story" -- in a nominally updated milieu. "Arthur" is one of those rare contemporary entertainments that can be used to contradict people who habitually complain, "They don't make 'em like they used to!" This time they have.

The movie's charm and assurance seem especially surprising when one considers the fact that Gordon had never directed a feature before an when one recalls his only previous screenwriting credit, the crass, hectic Henry Winkler comedy "The One and Only." Gordon has contributed dozens of scripts to television -- "Barney Miller," Chico and the Man," "The New Dick Van Dyke Show" and "The Practice," among other credits -- but he first made a reputation directing humorous commercials for a New York ad agency. It's conceivable that the deft, buoyant style that seems to emerge from nowhere in "Arthur" actually had its origins in his commerical training.

Gordon doesn't attempt complicated or ostentatious comedy effects. "Arthur" is a compact, intimate show, basically dependent on Gordon's ability and snappy dialogue. I suspect the script made good reading. Presumably, it's been enhanced immeasurably by smart casting and observant direction. Gordon often enhances simply presented situations with incisive timing or felicitous changes of camera angle. For example, there's a marvelous cut to the shocked reaction of Barney Martin, playing Linda's lumpy, unemployed dad, when she spurns Arthur's offer of financial assistance during a telephone conversation.

However, when it comes to impeccable timing and delivery, nothing in "Arthur" can surpass John Gielgud's explosively funny performance as the admirable Hobson, Arthur's sarcastic valet -- and lifelong nanny. Although Gordon has supplied several members of the cast with delightful material and flattering direction, he may have earned a special place in theatrical history by providing Gielgud with a great comic valedictory.

Unitl he's persuaded that a spark of slef-respect flicker sin Arther after all, Hobson treats his "master" with withering disdain. Even Eric Blore treated Edward Everett Horton with more deference. When Arthur remarks that he hates waiting in his father's office, Hobson typically cracks, "Of course you hate it. People work here." On the other hand, Hobson tolerates no wise stuff from outsiders. When an office employe makes a disparaging remark about Arthur, Hobson squelcehs the inteloper, and part of the comic impact of the squelch derives from one's giddy shock at hearing someone as austere as Gielgud resort to profane slang.

Although the character is sentimentalized in the closing stages (Hobson's health is precarious), Gielgud's dryness prevents mawkishness. The sentimental twist seems worth it when Moore attepts to cheer up his hospitalized mentor with a succession of treats and toys, climaxed by the presentation of a cowboy outfit. The sight of Gielgud in a Hopalong Cassidy hat is a memory to cherish. Even more so when he wearily instructs Moore, "If I die, kindly remove this thing from my head." If the Motion Picture Academy has any smarts -- and its neglect of Robert Morley's performance in "Who Is Killing the Great Chefs of Europe?" leaves plenty of room for doubt -- the membership ought to resolve that this year's Oscar for best supporting actor rightfully belongs to John Gielgud.