The hottest one-man show in Maryland opened yesterday afternoon as playwright Ernest Thompson returned to his hometown of Westminster.

In designer jeans, Wellington boots and white plastic, mirrored sunglasses, the 32-year-old author of "On Golden Pond" and "West Side Waltz" brought a day's worth of silver-screen panache to the placid exurb (pop. 8,800) 50 miles north of Washington where the barnsides still urge motorists to "Chew Mail Pouch Tobacco," where parking on Main Street costs 10 cents for two hours and where business as usual came second yesterday to the importance of seeing Ernest.

"I was here 20 years ago," Thompson told local fans and press at Western Maryland College, where his late father, Professor Theron, was head of the education department, "and I still can't believe that I'm older than the students here."

Born in Vermont, Richard Ernest Thompson (he dropped the first name when he became a soap opera star in the '70s) attended junior and senior high school here. He was a "real regular guy who kidded around with everybody," according to classmate Eddie Thomas, 32, who remembers Thompson as a cross-country runner with no discernible thespian yearnings. Ditto for Michael Eaton, the now-retired high school teacher who taught Thompson English. "How could you possibly know?" Eaton said. "Although I always saw the originality there."

Thompson said, "I think I was born in 1971 when I got out of college and thought seriously about show business." He got into it by accident as a student at the University of Maryland, when he and a friend tried out for parts in "Ah, Wilderness." The friend got the part, Thompson did not, and "I was so angry that I took an acting class." He had been advised as early as high school to capitalize on his matinee-idol face and FM announcer's voice. He transferred to American University and received his first major role in a play at Catholic University, beginning a seven-year dramatic odyssey that would take him through the NBC soap opera "Somerset," other network dramas and stage engagements with such stars as Alexis Smith and Jean Stapleton.

Then suddenly in 1977, he said yesterday, "I was rudely introduced to the notion of unemployment" and began writing out of "creative frustration." Not surprisingly for a person who had written a novel at 12, he quickly turned out three one-acts called "Answers," which CBS cable is filming now with stars including Burgess Meredith and Ned Beatty. A full-length play called "Lessons" followed, and then in '78, a play called "On Golden Pond."

During a number of hometown events yesterday including talks with WMC students and appearances at two benefit showings of "On Golden Pond," Thompson was reluctant to say to what extent the play was based on his parents, who had a summer cottage in Maine where Thompson's mother still lives in warm months. "They were the prototypes," he said. "And there was a lot of dialogue I stole from around the house."

But Westminster natives believe the resemblance is much closer. Norman Thayer closely resembles Thompson's father, said Eaton. "He was a real Yankee, and very hard to talk to."

And Thompson's mother, Esther, who had flown up from her winter home in Florida--and was the only person in Westminster with a deeper tan than her son's--said, "I think Mrs. Thayer resembles me quite a bit. I'm peppy and lively and quite an outdoors person." Her enthusiasm for picking wild strawberries, she says, became part of the play, as did her nagging of Mr. Thompson to spend time outside. "He was the real Norman Thayer Jr., she said. "He was more fearful of the future than I."

Generational conflict is the theme of "On Golden Pond," and perhaps of Thompson's earlier life as well. "I wrote 'On Golden Pond' before my father took ill" with cancer. "I write ahead of myself, maybe I would have liked that reconciliation with my father. At any rate I was better prepared to deal with him when he was dying" three years ago.

The same generational tension informs "West Side Waltz," which opened Wednesday at the Kennedy Center and stars Katharine Hepburn. Thompson complained that many theatergoers regard the play as simply a vehicle he wrote for her. "Actually," he said, "I'd never seen a Katharine Hepburn movie until 'On Golden Pond.' " He never followed vintage movies as a young actor, "and I was never that crazy about her." Moreover, he said, "She warned me she would distort it and change the balance."

In fact, he said, the senior female figure in "West Side Waltz" was inspired by a local Westminster figure of the same last name, playwright/director Dorothy Elderdice, the doyenne of Carroll County theater before her death from cancer in 1979 at the age of 87. Although not based directly on her life, Thompson said, "her spirit is there--her memory has inspired me to keep going. Unfortunately she was comatose before I could say that not only had I ripped her off, but that Katharine Hepburn would be playing her part."

In an afternoon ceremony in the converted church that is now the Carroll County Arts Center, Thompson presented the Dorothy Elderdice Community Theater Award to local dramatists Jane Davy and Arnold Hayes. He addressed a crowd of about 100--"I see my former dentist out there and it makes me nervous"--including his brother Paul, 35, a White House aide, who said his brother's fame had made it "easy to talk to people."

As Thompson meandered across the linoleum signing autographs on everything in sight, including the paperback of "On Golden Pond" ("They wanted me to write a novelization, but I find the whole concept repugnant"), local listeners kept waiting to hear what other influence Westminster years had had on his work. They had been disappointed earlier when he said, "You can't find them."

"But Westminster will get its moment," he said, as soon as he finishes working on a movie script tentatively titled "Kid Stuff," which he is writing in his Los Angeles home, and the novel he's writing and as soon as the first production of his play "A Sense of Humor" is scheduled.

"I'm working on a couple of pieces that are centered around Maryland. There's something so . . ." and here he paused, grappling for a term, ". . . so wonderfully neutral about Maryland. It's like Switzerland or something."