A COUPLE OF years ago, while rehearsing in the Stratford Festival's production of "As You Like It," Maggie Smith, as Rosalind, came to the moment at which she says she will be "more giddy in my desires than a monkey."

"There was an obvious desire to make it funny," recalls Robin Phillips, the festival's director, "so I told her, 'Just go flat out, Maggs. Climb the trees! Swing from the branches!'"

Maggie Smith went "absolutely crazed," climbing the trees and swinging from the branches before finishing her speech and collapsing at the center of the stage, Phillips remembers.

An astonished cast and crew went to their coffee breaks in admiring silence.

"The thing was," says Phillips, "that we had no tree in that studio. Yet we all knwe that she had climbed it. It was dazzling display."

"What sets Maggie Smith apart," remarks Michael Caine, her co-star last year in "California Suite," "is her ability to go from very high camp right down through every level of the theater. She can do it all. Her versatility is stunning."

"She is the only actress I've ever worked with," declares veteran film director George Cukor, "who made me ruin a scene by making me laugh out loud."

Given these accolades, one might expect Maggie Smith to be a comic zany turning cartwheels in the street. But in person, she is no laughing matter; once offstage, her demeanor cools rapidly.

Preparing to leave the Minskoff rehearsal studio in New York, the 5-foot-5, 44-year-old gem of the British theater looks glum. A few minutes before, she has completed another run-through of her acclaimed London performance as Ruth Carson, the lusty deflator of foreign correspondents and their journalistic pretentions in "Night and Day," Tom stoppard's new play which opens at the Kennedy Center Wednesday night.

Suddenly, the play's steamy African setting is gone, and so is Ruth Carson. Only Maggie Smith remains.

Dressed simply in a black pantsuit and white blouse, her bright green eyes mostly hidden behind pink lenses, she stands stiffly at the elevator, like a deposed monarch faintly aghast at the prospect of further indignities. She is girding herself to face that intermission in her life that must be played without a script. In a few minutes, she will have to talk about Maggie Smith. Nothing bores her more.

Not even a chance encounter with a friend can lift her spirits:

The elevator doors slide open. From within, a slim blond woman with a familiar face looks up and spots the next passenger:

"Mahgee!"

"Mia."

"We only run into each other on Saturday," exclaims Mia Farrow, each excited syllable precisely articulated in pixie British.

Her own rehearsal has just ended upstairs. As the two of them ride down together to the lobby, comparing out-of-town schedules before parting at the entrance, Farrow smiles frequently. Smith doesn't.

A short time later, after elegantly weaving through the sidewalk traffic along Broadway, Maggie Smith settles back in a pale green and pink chair in her producer's office, cigarette firmly placed in her fingers, and tries to explain:

"I don't want to talk about how I act because I can't, and I don't particularly like messing with it."

"You can't really get into it anyway," she adds. "You can't write it in hundreds of books."

She will say that "it's bloody hard work. The hardest part is the discipline needed to do eight performances a week. That's a lot of concentration. You always have to be at the theater at the right time. I mean, what if you weren't?

"People have a strange theory," she continues, "that actors and actresses are unreliable people.In fact, they're some of the most reliable people in the whole world."

Cukor says that Smith's reticence about her art simply proves that she is a "very, very intelligent actress."

"There's so much artsy bull -- from actresses shooting off their mouths about acting. I just think it revolts her. I myself suspect people who talk it all the time. Maggie just does it."

She's been acting professionally now since 1956, when an American producer noticed her in a revue at the old Watergate Theater in London. She was barely removed from an adolescence spent around Oxford theaters, and the producer took her to Broadway in "New Faces of 1956."

As a "mock-follies girl" in that show, she descended a staircase draped in enough oranges to sink a Tropicana boat. But she must have been impressive: Once back in London, she received her first film offer and began to work regularly at the Old Vic.

In 1963, Sir Laurence Olivier invited her to join the new National Theater Company of Britain. Shortly afterward, she was playing opposite him in Shakespeare's "Othello." The sequence of events still strikes her as miraculous.

"The directors there saw something in me -- mercifully -- and I suppose they took a great risk. You don't often put a light comedienne in 'Othello,'"

Smith's Desdemona eventually led to her first Academy Award nomination -- she has received four -- for Olivier's film version of the play. Ever since, her career has been a string of successes, even in properties soundly lambasted by the critics. Richard Burton, for instance, accused her of "grand larceny" for winning raves in the otherwise forgettable "The V.I.P.s."

After several remarkable small coups in B films like "Hot Millions" and "The Honeypot," Smith secured the film role of Muriel Spark's mercurial Scottish schoolmarm, Miss Jean Brodie. She shaped it so spectacularly that the 1969 Oscar seemed the least a grateful film community could offer her in return. On the first day of 1970, Queen Elizabeth II made her a Commander of the British Empire.

She married Robert Stephens, her co-star in "Jean Brodie." A few years ago, they were divorced. She has since married playwright Beverley Cross.

She and Cross live with her boys, aged 8 and 12 -- and her brother, sister-in-law and nephew -- in a big house in the Surrey countryside.

She is able to live comfortably, the film roles have come frequently enough -- "it's nice to be able to pay the school fees" -- and "California Suite" brought her a second Oscar. The stage accomplishments have remained undiminished.

At the Stratford Festival, to which she will return next season, and at theaters from London to Los Anleles, she has interpreted the characters of a wide sweep of playwrights. Now, for the first time, Tom Stoppard joins the list. s

Smith took over the role of Ruth Carson in London from Diana Rigg when Rigg left for another commitment. Smith plainly loves the play, which brings her to Washington for the first time.

"It's wonderful language," she says, the first noticeable trace of a gentle laugh rolling in her throat. One pleasure rests in Stoppard's instruction that certain of Ruth's thoughts are to be made audible to the audience.

"It's unusual and marvelous to play somebody who speaks her thoughts. It's rather nice to declare your fantasies."

That, in a way, is what she did in "California Suite." But acting the role of an actress can be double trouble. The character of Diana Barry -- a British Oscar nominee in the movie who frets incessantly on the night before the ceremony -- tested Smith's professional and personal resources.

"Yes, it was very weird indeed," she says pointedly. "I had to more or less be me, which was very -- disquieting."

"It was very, very hard for her," recalls Michael Cane. "She used a tremendous amount of her own, shall we say, idiosyncracies."

"The balance in the part was difficult to get right," says Herbert Ross, who directed the film. "It had to be funny, but it hit on a lot of the insecurities that any actress has."

Maggie Smith, naturally, brought it off. Although she was driven almost to drink by the series of mirror images -- culminating in her own "reliving" of the film when the Academy nominated her for playing Diana Barry.

The airline she took to California for the ceremonies had been showing the film on its flight to the coast. "Mercifully," she says, filling her favorite word with extra power, they didn't show it on her flight.

"I would have been totally unhinged if that had happened."

Despite Phillips' observation that "her nerve ends are very close to the surface," Smith seems very firmly hinged. Her discomfort with what she once called her "dreary" looks remains present, but under control like everthing else. What matters most and always for Maggie Smith is acting -- as evidenced by her planned return to the Stratford Festival.

"Everyone knows that she comes here at a great financial loss," says Phillips, who praises Smith's unstinting concern for her fellow actors, great and small. "The commitment is the most colossal thing and the most important thing."

For Maggie Smith, though, the colossal thing is to stretch herself even further. She sounds determined to climb a few more trees.

"I started off being put in a pigeonhole so early in my career that i've fought tooth and nail to get out of it. Nothing's going to put me back in it."