OUTSIDE THE boundaries of the British Isles, Bill Forsyth, the writer/director of "Gregory's Girl," is the Scottish film industry. His two movies -- "That Sinking Feeling," which was filmed in three weeks during 1978 on a budget of $14,000, and "Gregory's Girl," shot, Forsyth says, "between raindrops" in 1980 at a cost of $400,000 -- are the first to be produced wholly in Scotland and entirely by Scots. "Gregory's Girl" won Forsyth the British Academy Award for Best Screenplay, over competition from "Chariots of Fire," as well as nominations for Best Picture and Best Director.

The acclaim for "Gregory's Girl" has prompted a partnership between Forsyth and "Chariots of Fire" producer David Puttman, which has allowed Forsyth to begin work on his latest project, "Local Hero," starring Burt Lancaster. Forsyth describes the film as "a combination of 'Apocalypse Now' and 'Brigadoon,' " to be filmed partly in a rustic fishing village in Scotland and partly in Houston, Tex.

" 'Local Hero' doesn't have any gushers or fires or elaborate pyrotechnics," he says apologetically. "I couldn't do that. I don't think that I could handle action at all. Anyway I'd be bored with it. Technically it sounds like it would be an awful lot of hard work. It's much easier just to get some people and have them sit down and exchange a few words."

It is precisely this philosophy that has carried "Gregory's Girl" to an audience of respectable, though not record-breaking, size. The movie has grossed $1.4 million to date, and went into multiple release in the Washington area a week ago. "We've been very, very happy with the gross, especially going up against the big summer pictures," said a spokesman for the distributors, Samuel Goldwyn Co.

Forsyth says the script for "Gregory's Girl" was an irresistible scenario the British Film Institute had been able to resist for the two years prior to the making of "That Sinking Feeling." After the film's success, they reconsidered their position and granted funding for "Gregory's Girl."

The story of "Gregory's Girl" is a simple one. It focuses on a 16-year-old boy named Gregory, played with gangling charm by Gordon John Sinclair, a good-natured flake confused by the curves his rapidly changing body is throwing him and his sudden, perplexing introduction to the world of women. The story is set in the city of Cumbernauld, near Glasgow, and stars most of the cast from Forsyth's first movie, all from the Glasgow Youth Theatre.

"Cumbernauld is a satellite town outside of Glasgow that was constructed about 20 years ago to ease the population problem there," Forsyth says. "However, in the last 15 years, Glasgow has become a sort of post-industrial wasteland. The population has dropped from 1.1 million to 600,000. Now there is no population problem and no real need for cities like Cumbernauld."

Forsyth says, however, that the town was ideally suited to his purposes.

"The age of the city allowed me to make everything in the film reflect adolescence," he says. "Everything, the buildings, the streets, the trees, are all about 17 years old. The town itself struck me not as modern or futuristic, but gawky and not yet settled in its way."

"In the same sense," he continues, "there are very few grown-ups in the film. During that time in your life, your vision is sort of distorted and, because of your own needs, you have a blanket impression of things. As a result, your parents and other adults recede into the background almost as if they had disappeared."

"The main idea which I wanted to convey was that the things which occur in adolescence continue on throughout the rest of your life; that adolescence never stops, and the wheeling and dealing that adults do is basically the same nonsense they did as kids. The situations in the film are basically negotiations, trading one thing for another -- you do this for me, and I'll do this for you. I wanted to reflect the adult world and the politics of living. The fact that the girls are in charge is just a slight dramatic distortion because of Gregory's point of view. We have heightened it some in the film, but I think, to a large measure, these facts are true in life."

Boyishly freckled with dark hair and robin's-egg blue eyes, the 33-year-old Scot is dressed in a sky-blue leather sport coat, a red Scotch-plaid flannel shirt, and sand-colored slacks. He speaks with a thick but lilting brogue, his voice, which has to compete with the incessant noisy clatter of the restaurant on Manhattan's Upper West Side, never rising above a measured hush. His introduction to the movie world, he explains, came at 17 when he answered an ad in the local Glasgow newspaper.

"At the time I was just out of school and looking for work," Forsyth recalls. "I didn't have any particular interest in filmmaking and really never much went to the movies. It was a trainee job, like an apprenticeship, with a one-man company, and it seemed like a job more interesting than most."

Forsyth says his first experiences as a filmmaker were less than romantic.

"We made industrial films," he explains, "very basic stuff. The boss shot and edited them and I carried all the stuff."

Forsyth was admitted to the National Film School in London in its first year, but soon thereafter, he says, for personal reasons, returned to Glasgow, formed his own film unit and waited for something to happen. For the next five years he produced television commercials and industrial films but admits "there wasn't much joy in it. It was just a living."

"During those years we would dream about getting the money to make a feature," he remembers, "but we saw feature films as being something at the end of the rainbow. We had a very passive attitude then. Not knowing any better, we thought that someone might see some film we had done on forestry and say, 'Why, you're just the lads to make my next feature.' It was cuckoo actually. In '77, I began to make some moves of my own."

His actions resulted directly in the beginning of his work with the Glasgow Youth Theatre and the production of "That Sinking Feeling." Financed by a cooperative of crew members and friends, the film went on to receive wide appreciation and favorable reviews at the Edinborough and London film festivals, and was eventually shown on Scottish television.

"I had really made the film just for the kids," Forsyth says. "I had expected that it would be shown in their clubs or in school. We only had enough money to buy the film we needed and have it processed. We shot it entirely in the streets of Glasgow and for interiors we used a plumber's warehouse."

In fact, the plumber's warehouse figured quite prominently in the plot of the film. Because of widespread unemployment among teens, a group of young Scots plan an elaborate robbery. However, the only place with anything of value is the warehouse from which the mob burgles 90 stainless steel sinks.

" 'That Sinking Feeling' was a film about the kids and the way they felt about their lives," Forsyth says. "During the filming I suddenly found out that, even in the few years that separated me in age from the cast, life for adolescents had changed a hell of a lot. When I got out of school there were pages and pages of ads for jobs every day. I found out that today 16- and 17-year-old kids were coming out of school with absolutely no future at all. It was a film about kids with nothing but time on their hands and who were simply trying to find a way to fill it up. The fact that the heist was so elaborate gave away the fact that they were doing it more for the sake of doing something than for any sort of gain."

Forsyth says he believes the acclaim for the film came primarily because of its origins. "I don't think that the film was popular so much because of its particular merits," he says, "but simply because it came out of nowhere. All of a sudden there was a film from Scotland -- nobody had heard of such a thing. If I had been a filmmaker from London, I wouldn't have received half the attention. As it was, however, I now had a handful of good reviews which I could wave in everyone's face and say, 'Hey, look, you gonna help me make another one?' "

Working with the Youth Theatre in Glasgow, he says, "was an excellent way for me to learn how to work with people. It was difficult for me at first because I'm not an extroverted person, and I shied away from directing for that reason. I presumed that the people who directed films were extroverted and took charge of things. It was such a nice experience just getting to know these kids and finding out that my presumptions were wrong."

Forsyth will abandon the Glasgow Youth Theatre to make "Local Hero" but will not abandon his homeland and can foresee nothing in the future that should tempt him to do so. He explains that the chance to make movies on a larger scale with bigger budgets doesn't interest him.

"The film I'm working on right now costs about $6 million," he says. "It's really by today's standards not that much money for a feature, but it's enough for me. I think that this amount is about the limit. If you go beyond that, for example, to a budget of $10 million, then you relinquish a lot of your freedom."

"I don't think I would be happy making movies in Hollywood," he continues. "In the business there the 'package' is everything, rather than the aspirations of the filmmaker. I don't want to discuss with a dozen people why 'Star A' is better for a part than 'Star B' when it was written for 'Star C' in the first place. If that's what it means to work in Hollywood, I'd just rather not get involved in it.

"I would much prefer to stay to Scotland. Naturally I feel more comfortable there, and right now the opportunity to put projects together is good. So," he says blithely, "there is no real reason for me to leave."

"Besides," he offers finally with palms up and in perfect deadpan, "no one's asked me to."