For years, this is what actor Jaston Williams -- 32 years old, 5-foot-5, hippie-tressed and thin as a knife at 120 pounds -- found himself playing: Puck, Ariel, gargoyles, spirits, psychopaths and The Hunchback of Notre Dame.

This is what actor Joe Sears--5-foot-10, 33 years old, slightly beady-eyed and portly as a dinner roll--found himself playing: older men, Bottom, Thisbe, doctors and senators.

"I've always looked older than I am," admits Sears. "When I went to the movies as a kid, I'd have to bring a note from my mother saying 'Little Joe is not yet 12 years old.' Otherwise, they charged me adult prices."

"We were pushed into character roles before our time," adds Williams.

That is more or less why the two of them, with the help of a director friend, Ed Howard, set out to write "Greater Tuna," the spiffy little satire of backwater mentality and mores in the third smallest town in Texas that opened last night in the Terrace Theater. When the agents and directors have you pigeonholed, actor, cast thyself.

Sears and Williams ended up giving themselves 10 roles apiece (if you count Yippy, the dog) in the quick-change comedy for which they constitute the entire cast--roles, it is fair to assume, they never would have played otherwise:

Like Petey Fisk, the sweet twerp who runs the Greater Tuna Humane Society and knocks himself out trying to save the town's mutts, strays and even the ducks. (That's Williams.)

Or big, bosomy Aunt Pearl Burras, the noncomformist, who poisons the very dogs Petey's out to rescue--"I'm going to get me a poodle," she chirps, gleefully--at the same time she tries to save her misfit nephew from his wayward ways. (That's Sears.)

Or Charlene, the large-hipped teen-ager who can't get herself elected cheerleader, and who informs God indignantly, "I've prayed to you for seven years to be cheerleader--three in junior high school and four in high school, and NOTHING!" (Williams again.)

Or Reverend Spikes--a failed Ernest Angley, who wants to rally the town against smut and rock 'n' roll records, with the exception of Elvis Presley's because Presley was, of course, a good Southern boy. (Sears.)

Or Bertha Bumiller, the book-burning housewife who dislikes "Roots" because it shows "only one side of the slavery issue"; Hank, her honky-tonk husband, who's never home; Arles Struvie, the folksy radio announcer at station OKKK (250 watts); demented Stanley, the teen-age tough marked for reform school, and tender Nadine Wooten, Tuna's Delta Dawn, who stands on the highway every fall, waiting to no avail to be picked up by the man who jilted her years ago. All of them ineffably narrow-minded, funny, colorful and, in the double-barreled interpretation of Williams and Sears, as distinctive as the Texas mesquite.

"We ended up with a lot of misguided people, who were bigoted to the bottom of their toes," notes Sears. "But just because someone is misguided about something doesn't make him a bad person. That's what we've tried to point out. These are our backgrounds, the people we saw growing up."

"We do have an abiding love for them, in spite of their failings," agrees Williams. "Oh, I wouldn't want to live there again, but I'd like to think I could go back and know I hadn't hurt anyone's feelings."

Williams was raised in Crosbyton in the West Texas panhandle, Sears in Bartlesville, Okla.--and other than Bartlesville being bigger than Crosbyton, they seem to have a fairly common pool of experience. "Our surroundings were similar," says Sears. "I remember all those characters, the hay hands who worked for my grandfather, the lady down the street and how she walked, or my Aunt Pearl, who actually poisoned dogs with what she called 'the bitter pill.' She wasn't mean, but it was during the Depression and people would dump dogs on the road. Hers was the closest house and she raised chickens, and the dogs headed right for her place. She didn't want to use a gun, so the bitter pill was developed."

"That whole bit about Charlene wanting to be a cheerleader and praying mightily for it comes out of an experience when I was in high school," says Williams, "and I had to go to Wednesday night prayer at the Methodist church. And, thank the Lord, Debby V---- finally got cheerleader, because she'd been praying for it every Wednesday night for seven years. She and God were pretty tight."

Williams also remembers the prototype for Stanley: "There was always a kid in every small town who got sent away to remind everyone else in town of the threat of reform school. They'd just take some poor sucker and schlep him out. Of course, I wasn't allowed to hang out with those types very much, because my mother was a schoolteacher, but I'd hop on the back of Larry S----'s motorcycle nonetheless and raise hell with them the best that I could."

Being inveterate storytellers, Williams and Sears kicked around the material for some time at parties before they actually decided to develop it into a show that follows Greater Tuna's populace from sunup to sundown. They had both been hired by Trans Act, a commercial theater in Austin, launched with great fanfare and what the staff thought was five years of guaranteed financing. Four months after it opened, however, Trans Act folded and they were on the street.

So with a $10,000 bank loan taken out by their director, Ed Howard, they turned around and rented the very theater that had fired them, wrote "Greater Tuna" and moved back in. That was in the winter of l981 and "Greater Tuna" has edged onward and upward ever since, gaining new characters and refinements in such cities as Atlanta and Hartford. Last November, it settled in to the Circle-in-the-Square in New York for a run that continues merrily with replacements, now that Sears and Williams have taken to the road again.

Norman Lear has purchased the television rights and wants to develop it into six half-hour shows. To that end, Williams, Sears and Howard are creating new Tuna citizens (35 or 40 is their goal) and writing a teleplay. Lear is considering the series for cable television next spring, not because the content is all that controversial, but, as Williams notes, "When someone in Texas says 'Goddammit' with conviction, he's got to be able to say it."

"It's not sacrilegious," adds Sears, "it's idiom."

For all its rarin' good fun, "Greater Tuna" does score points at the expense of small-town conservatism, the Moral Majority and the tight grip of the Bible Belt. If Diane Arbus had written "Our Town," instead of Thornton Wilder, she might have come up with something similar to the Sears/Williams/Howard collaboration.

"I guess you could say that we've taken bodies we've seen in our home towns and put into them the views of eccentric Moral Majority types we read about in the newspapers. We've compacted them," explains Sears. "Each town seems to have these people who want to manipulate other people. It's impossible to take them seriously, but the trouble is in most cases they are serious."

"I remember once we were going to put something in the show about taxing welfare mothers and prisoners," says Williams. "But we thought that's too stupid. Nobody will buy that. Oddly enough, three weeks later it was proposed right here in this very city of Washington ."

"I was raised in an area in which religion was such hypocrisy," continues Sears. "What I saw on TV and heard in church and saw on the streets were completely different. Whenever people from the bottom rung of the ladder came into the First Baptist Church and sat in the back row, it was sure to be the talk of the whole congregation at teatime. That always confused and bothered me. My grandmother sent money to Oral Roberts for years and years and years. Well, to a logically minded person, it just seemed that an advantage was being taken.

"In the show," he continues, "we came up with Reverend Spikes, who might have been a pretty good country preacher once. But he represents all those failed preachers who have to keep up on the faddish issues in religion or else their congregation will send them packing. When we were first working on Tuna, there were those rock 'n' roll record burnings in Waco, Texas. Before we got finished with the script, they had already spread to several other towns."

"Small towns can be very repressive," agrees Williams. "I was too tiny to play football. I played basketball, which was real silly of me. Got hurt all the time. But there was nothing I could do, basically, to feel good about myself or feel that I was going somewhere. Being a small person doesn't count for much in West Texas. I always felt like an observer."

To their delight, Sears and Williams have discovered that their observations of rural Texas seem to strike a familiar chord with audiences from all parts of the country. Take away the twang and cowboy boots and you could, after all, have Hicktown, Maine, or Piffle Falls, Ga. "We didn't want to do caricature," says Sears. "That would be so demeaning. I'm an actor that likes to work with detail and I was not at all convinced that we could play the women characters and do them justice. Jaston pressured me into thinking that we could. Curiously, that turned out to work in our favor. We put such emphasis on the women that our men seemed lacking. So we expended even more effort on the men. I just recently got the husband Hank, who makes a quick little entrance. It took me a year to convince myself that I could be a sexy man. Now when I put on his cowboy hat each night, I believe."

"One lady arrived late for the show when we were playing in Hartford," Williams volunteers. "She didn't realize until intermission that it was two men playing all the characters. I guess that was the greatest compliment we ever got."

Success has come to the two so rapidly that they still project a starry-eyed wonder at the magical ways of show business and a kind of aw-shucks modesty, bordering on sweet naivete. They admit frankly they were "scared to death" to play New York--Williams had never been there before--and now talk about the Kennedy Center with the reverence that a French peasant might bestow on Versailles or some such temple of magnificence. They've heard tales about devious producers and unscrupulous agents, but they hasten to add that everyone they've met in the business so far has been "just as nice as they could be."

The monetary rewards have been unexpectedly pleasant, too. Williams says he can now afford to put his 12-year old son into private school in Texas "to save him from the education I didn't have. And my mother told me she could die in peace. If nothing else, she's no longer worried about me financially. Nobody in the family is saying, 'Be a florist, get a master's.' Get a master's? Hell, get a bachelor's."

As for Sears, he has finally wiped out the ruinous debt he ran up with his credit cards in Texas. "I was one of those who freaked out on credit cards," he admits sheepishly. "I had credit cards you couldn't imagine. Well, I ripped them all up one day in the park. Jaston was there."

He beams. "Now I bank."