Review/Theater; Rex Harrison Back on Broadway

When Rex Harrison makes his long-awaited star entrance, the evening's last, in the Broadway revival of ''The Circle,'' one can't help wondering: Why does he need to do this?

With more than a half-century of memorable roles behind him, Mr. Harrison hardly must prove himself as an actor. Nor, at age 81, is he likely to show the audience anything new. But here he is on stage at the Ambassador Theater, regal in an Edwardian cream linen suit, playing his part in a 1921 W. Somerset Maugham comedy that seems much older than he is. And Mr. Harrison chooses to undertake this exertion eight times a week.

It doesn't take forever to figure out why. Mr. Harrison, looking fitter than he has in other Broadway outings of this decade, clearly still delights in performing. ''The Circle,'' which he has hinted in interviews will be his valedictory, lets him strut as much as flesh and bones will permit. As Lord Porteous, a once-promising Member of Parliament whose career ended in romantic scandal and exile 30 years ago, Mr. Harrison gets to grind his ''damn new teeth,'' bray insults at his intellectual inferiors and in general carry on as irascibly as ever. Peering across the sitting room at his pompous host's prized, newly bought antique chair, Mr. Harrison raises spectacles to his eyes and immediately declares, ''It's a fake!'' One hears the same gleeful contempt with which Henry Higgins once dismissed the Hungarian phonetician who crossed Eliza Doolittle at the Embassy Ball.

Yes, Mr. Harrison carries on. So do the somewhat spryer co-stars who have joined him for this occasion, Glynis Johns and Stewart Granger. As for ''The Circle,'' it is merely a convenience that allows a devoted audience to enjoy three troupers enjoying themselves doing what each does best. The result is a theatrical experience of a very specific type, to be sure, but an entirely valid one. Just be certain you understand what you're getting into.

From Desmond Heeley's country-home set, whose sea-green walls almost seem to have acquired a patina of algae with age, to Brian Murray's staging, which consists mainly of draping the actors around the furniture, ''The Circle'' is a throwback to a distant era in West End boulevard theater. The production almost makes the last Harrison vehicle -Frederick Lonsdale's ''Aren't We All?'' (1923), seen in 1985 - look avant-garde. It's the kind of relic best encountered on a chilly, rainy, late autumn afternoon; Maugham's writing all but demands the counterpoint of matinee patrons removing their galoshes and adjusting their holiday shopping bags.

The dramaturgical geometry of ''The Circle'' actually involves two love triangles. Lord Porteous has traveled from Italy to England with Lady Kitty Champion-Cheney (Miss Johns), the married woman for whom he so famously sacrificed his career. As the couple have their reunion with Lady Kitty's son in Dorset, they not only must confront the husband they long ago cuckolded (Mr. Granger), but they also happen upon a new scandal in the brewing. Kitty's pompous son (Robin Chadwick), himself now a rising M.P., may soon lose his wife (Roma Downey) to a sweet-talking tennis-playing house guest from the Malay States (Harley Venton).

Will family history repeat itself and come full circle? I'll hold my peace. The answer would be more interesting, in any case, if the author had something enduring to say about marriage, infidelity and love, or if he expressed his perishable views more wittily. Its occasional laughs and solid construction notwithstanding, ''The Circle'' isn't about to slow the decline in Maugham's literary reputation since his death in 1965, and it hardly leaves one eager to exhume the 20-odd other plays (''The Constant Wife'' most famous among them) that he wrote before abandoning the theater in the 1930's. The flat boilerplate dialogue - ''Do you know that I'm awfully in love with you?'' or ''You can't serve God and mammon!'' - helps one appreciate how excited English audiences must have been when Noel Coward refitted similar sitting rooms with ''Hay Fever'' and ''The Vortex'' only a few seasons later.

The most tiresome burdens of ''The Circle'' fall on the younger characters, here played by decent actors who have been encouraged to accentuate the antiquity of the piece by striking anachronistic, at times campy stock-company attitudes. Their elders play for keeps. Miss Johns, the one member of the company called upon to convey emotion, is a poignant figure: a once-vibrant woman who has been transformed into a silly painted chatterbox by a frivolous life. When she and Mr. Harrison bicker about the disappointments that eventually turned their passionate illicit liaison into a sour parody of marriage, one feels her bitterness even as one exults in the verbal chamber music created by two of the most distinctive speaking voices in the English theater. A later scene, in which Miss Johns confesses to her daughter-in-law that her ''smiling eyes'' hide an ''aching heart,'' mists over in affecting tears.

Mr. Granger's job is to be a charming consort, and he executes it most handsomely. He also gets to deliver the script's most boisterously received line: ''I suppose it's hard for the young to realize that you can be old without being a fool.'' Actually, it's not so hard as he might think, at least for those who have watched the trio of sly old foxes at play in ''The Circle.'' POSSIBLE VALEDICTORY OF REGAL IRASCIBILITY - THE CIRCLE, by W. Somerset Maugham; directed by Brian Murray; scenery design by Desmond Heeley; costume design by Jane Greenwood; lighting design, John Michael Deegan; production stage manager, Mitchell Erickson; general manager, Ralph Roseman. Presented by Elliot Martin, the Shubert Organization and the Suntory International Corporation. At the Ambassador Theater, 215 West 49th Street. Arnold Champion-Cheney, M.P....Robin Chadwick Mrs. Shenstone...Patricia Conolly Footman...Robertson Dean Elizabeth...Roma Downey Edward Luton...Harley Venton Clive Champion-Cheney...Stewart Granger Butler...Louis Turenne Lady Catherine Champion-Cheney...Glynis Johns Lord Porteous...Rex Harrison