An impressive lot has happened to "On Golden Pond" and author Ernest Thompson since its Kennedy Center run in early 1979.

Around the world, so far in 1981, some 150 productions of the play are scheduled. It's now on view from Florida's Asolo Theater to Alaska's Anchorage. Janet Gaynor is about to move into an already running Chicago production. Boston and Baltimore soon will see one headed by Sada Thompson and James Whitmore. Julie Harris acted it last year in Los Angeles opposite that admired actor, Charles Durning, and is hoping to resume the role of Ethel Thayer in San Francisco this spring. Durnings's standby, Wiley Harper, has been heading a cast of San Diego's Old Globe and Canada's leading stars, Donald Davis and Frances Hyland, also have acted the Thayers.

Closer to home, Virginia's Hayloft Dinner Theater is about to have its own production, this one headed by Pat O'Brien, his wife Eloise and their daughter Brigid, their same relationships in the play. In his 61st year as an actor, the veteran film star boasts that his 81 years make him closest in age to Thayer's 80th birthday than all the actors who have played him. O'Brien's uniqueness, however, could be challenged by Maurice Evans, who will be 80 come June, when still another company is mentioning him for the part.

Late fall will bring the film version, the three Thayers acted by Katharine Hepburn, Henry Fonda and Jane Fonda. Mark Rydell, who directed them last summer in a lovely New Hampshire setting, now is editing his reels meticulously on his Kem Bild-Ton system, a sophisticated outcropping of those old "movieolas," on which early directors and editors did their cutting and splicing. David Grusin is completing the scoring, which a full-scale orchestra will be taping in mid-March.

Although other recent plays have attracted vastly more critical attention, none has had such a vogue of worldwide facsimiles, reflected in clippings from Sweden, South Africa, Finland, Argentina and Germany. "On Golden Pond," introduced by Off-Broadway's Hudson Guild Theater, uniquely had two more New York opening nights, on Broadway and later at a "middle-Broadway" house, covering two years in all.

What is the key to all this activity?

As I wrote at its opening, this mellow love story is "richly comical on the surface, deeply moving below . . . its acceptance and affirmation of life not unlike the quality Thornton Wilder poured into 'Our Town.'"

What has all this done to Thompson, now 31?

He's hired a business manager to take care of all the deals his play agent has set up and he's kept to his typewriter. Besides writing the play's screen adaptation, he's done a script on the hospice movement for NBC producer Philip Barry Jr., and is working on a film for Robert Fryer, "Kidstuff," as if to underscore that his interest is not confined to senior citizens.

Two of "Golden Pond's" Norman Thayers, Fonda and Durning, see roles for themselves in "Answers," the three short plays Thompson wrote "to see whether I was a playwright" between acting assignments. They want to costar in a Los Angeles "Answers" this spring under the direction of Charles Nelson Reilly.

Now Katharine Hepburn is playing her second Thompson role, the life-loving Margaret Mary Elkerdice in the second Thompson script to reach a stage, "The West Side Waltz," continuing here at the Music Center's Ahmanson Theater.

Thompson had completed the play about three New York women, aged 70, 50 and 30, long before Hepburn came to "On Golden Pond." It's an architecturally balanced mood play about those anonymous ladies seen in the streets and apartment house lobbies in the Riverside Drive area of New York's West Seventies.

With as shrewd an eye for a role as anyone in the profession (which is why she's lasted so long), Hepburn quickly opted to play doughtly Margaret Mary. She even went back to the coach she'd had when she played the piano as Clara Schumann, in "Song of Love" 35 years before, to enhance the duets she plays at the end of all five scenes in "The West Side Waltz."

Her costar and duet partner, on the violin, is that deft and winning comedienne, Dorothy Loudon, who created the unforgettable Miss Hanrahan of "Annie," forming a highly novel teaming of feminine stars.

Hepburn's is a far better role than the starring part she had in Enid Bagnold's "A Matter of Gravity," her previous stage vehicle, and audiences welcome her with obvious relish, roars of laughter and standing ovations. The Ahmanson is selling out through March 14 and there are still 12 weeks to go in San Francisco, Seattle and Vancouver before Hepburn starts her summer holiday June 1 after one of the busiest of her 72 years. Touring will resume in the fall, with the Kennedy Center as one of the pre-New York stands.

Still, there is more of a play here than meets the eye and as Los Angeles Times critic Dan Sullivan put it, "this is a less slavish vehicle than many and . . . it would be nice to see Thompson's play with a mortal."

In other words, it is Hepburn's star presence that packs the theater for a gentle play that might otherwise get lost, but it is this same quality that can swamp the play itself.

As the 50-year-old violinist Loudon more than holds her own, but the casting of the other three roles is almost incomprehensible. The part of 30-year-old Robin literally is washed out by Regina Baff, whose diction lacks clarity and whose monotone gives one a toothache. Baff doesn't seem to have a clue of the role she is playing against the other women and the play's deliberate architecture thereby is utterly ignored. Don Howard, as her suitor, seems to have wandered in from an over-ambitious community theater and the often able David Margulies persists in leering as the Romanian building superintendent, whose appeal should be innocent delight in his new world freedom.

Whether director Noel Willman and co-producer Robert Whitehead (of Whitehead and Roger L. Stevens) determine to redress these and other balances before resuming in the fall remains to be seen.

At all events, Thompson is grateful for Hepburn's enthusiasm, as would be any playwright. He's completed another play, "A Sense of Humor," wholly unlike any he's done, and has specific ideas for future projects.

"Ever since 'Golden Pond' got rolling," he says, "I've been asked to adapt this novel to the stage, that play to the screen, all old ideas which have had their day. I'm not in the least bit interested in adapting a damn thing and I hope people will stop asking me."