IT WAS the day before former Bolshoi ballerina Valentina Kozlova was to make her debut in the Kennedy Center's revival of ''On Your Toes,'' and the young, handsome couple -- Valentina and her dancer husband, Leonid Kozlov -- were talking about their involvement in their very first American musical show.

Somehow, the conversation wandered onto the subject of a dancer whom Valentina had replaced at the last minute during a Canadian television project.

Valentina, strikingly comely in her olive leather jump suit, boots and a black turtleneck (Leonid wore a matching outfit in rust), her blond hair framing wide-set eyes, sculpted cheeks and a fashion model's features, smiled her most shrewd, alluring smile and exclaimed, "Enough of this--let's talk about pause , Me!"

The humor of the statement, with its mock egomania, was that it's a line from "On Your Toes," in the scene where the conceited Russian ballerina Vera Baronova (played by Valentina) flirts with an American vaudeville hoofer, Junior Dolan (Lara Teeter), in her bedroom suite. It underscored the fact that for both Valentina and her husband, learning their roles in the show was part of their continuing education in English.

"I have 45 lines in script," Leonid interjected, "and now I use all of them." Then turning to his wife with a pretend scowl, he jeered: "She is chelloosss!" (another line from the same scene spoken by Vera's ballet partner and lover, Konstantine Morrosine, the role Leonid will assume).

The couple's fluency in English, despite dropped articles and accents as pungent as caviar, seems astounding. Neither before their defection from the Bolshoi Ballet in 1979 nor since has either of them had any instruction in English, formal or informal. Asked how they learned the language, Valentina replied, "You know, we didn't learn it at all, we just tried to speak, with help of many American friends. In Russia, we had only French. Here, in this country, we had no school, no classes, no tutor--I do have little dictionary I carry with me."

Since this conversation, Valentina already has made her debut as Vera in the Rodgers and Hart musical, a week earlier than planned because of the unfortunate accident on the stage of the Kennedy Center Opera House a fortnight ago, when a piece of falling scenery fractured star Natalia Makarova's shoulder. Leonid, who will enter the cast as scheduled Tuesday evening, was in the audience at the time of the mishap and at first feared it might be fatal. "I thought it was finish for sure," he said. "This heavy thing came down, Boomph! She was so lucky, very lucky."

For Valentina, the early start augmented the challenge of performing her first musical comedy role, in a foreign tongue, her first choreography by George Balanchine, and in the footsteps of a celebrated predecessor. Was she nervous?

"Nervous? Of course, nervous," she said. "I have nerves--but, not panic. In the Bolshoi, I was always nervous opening nights, but never had panic, because preparation was so strong, so good. I was always sure of myself, even with nerves. Here, nobody teaches how to be star on stage, but in Bolshoi, it's different, it's the old Russian school. You can appreciate many other great artists, but on stage you have to believe, really believe, you are number one, you are the first."

As part of her preparation for "On Your Toes," she watched Makarova as Vera, and also Starr Danias, Makarova's understudy who took over the part for the week following the accident, but refrained from seeing them often. "I didn't want to be too influenced by what they did," Valentina said. "In the ballet, I can watch other artists many times, but for this, I felt it would be better to do my own way."

Both Kozlovs trained at the Bolshoi Ballet School in Moscow before joining the troupe and becoming principals.

Valentina, now in her mid-20s, entered the school at age 9, the jury's first choice from a field of 30,000 applicants. In 1973, when she still was a student, she joined a Hurok-sponsored Bolshoi tour to the United States--it was the trip on which she met her future husband, already a Bolshoi principal. In the Soviet Union and on many trips abroad, Valentina danced the full range of classic heroines in such traditional ballets as "Swan Lake," "Giselle" and "Don Quixote," as well as in modern Soviet classics, such as "Romeo and Juliet" and "The Stone Flower."

Leonid, 35, danced for 13 years with the Bolshoi, most of them as a principal in major roles.

Since their defection, the couple--who say they left in search of greater artistic freedom--has ventured into contemporary ballets by such choreographers as Alvin Ailey, Glen Tetley, John Cranko and Roland Petit; and Leonid has both staged and choreographed evening-length ballets for a number of companies internationally.

The Kozlovs speak candidly about their defection these days, though it is a topic they would rather play down, preferring to concentrate on their new lives and careers in the West. "We had thought about it for years," Leonid said, "but it was especially difficult at first because for some years I was denied a passport to tour with the Bolshoi."

"Then, in 1978," Valentina chimed in, "they needed us for a tour, and we wanted to do it then, but it was hard, it was a short tour only to small cities." The couple had no American contacts, and were keenly aware of the nervousness of Soviet authorities in view of the many previous, well-publicized ballet defections. "We just spoke about it at night, in whispers, only the two of us," Valentina recalled.

When the full Bolshoi troupe toured this country again in 1979, Alexander Godunov made headlines with a defection in New York that involved parting from his wife and a standoff at Kennedy Airport between U.S. State Department officials and Soviet mission people over his wife's departure. This only made the KGB more watchful, the Kozlovs assert, and their own departure at the end of the same tour more perilous.

Moreover, they wanted as little "scandal" as possible. "We had no quarrel with the Bolshoi," Leonid said. "It was terrible for us to leave, our friends, family, our teachers. And the last few years with the company had been extremely good ones artistically for us. We wished we could just say, thank you very much, but we want to expand ourselves artistically, and leave. Unfortunately, in Russia one cannot do this. So we waited until the last performance of the tour, in Los Angeles, and left as quietly as we could." Since the defection, the couple has been able to telephone relatives in the Soviet Union, but not friends: "It's too dangerous for them," they said.

One thing about the Bolshoi they don't miss is the factionalism that has split the company in recent years into bickering camps, and the contention over the directorship of choreographer Yuri Grigorovich. "It's one reason we left," said Leonid. "Grigorovich wants the dancers only to do Grigorovich ballets. We love the work of Grigorovich, especially his dramatic works, like 'Ivan the Terrible,' but that's only one side of the repertoire."

Valentina added, "If I was asked by Ladimir Vasiliev or Maya Plisetskaya or some other choreographer to be in their ballet, if the company chose me to be in one, immediately I am enemy of Grigorovich. It was impossible. You always had to worry about these politics, to play games with them."

Leonid noted also, "If you become a ballet star in Russia, that's very important--ice hockey and ballet are the important things to the Russians. And if you are a star, you can speak out, say things. But not too much, never too much--that's why I lost my passport those years."

The Kozlovs declare themselves extremely happy with their new life in the West, and especially with the choreographic diversity they've sampled. They have a home in suburban New Jersey (they are the artistic directors of the Classical Ballet of New Jersey in Closter, close to New York City), and see a rich array of options for the immediate future. They are scheduled to continue with the "On Your Toes" production to Seattle for a month's run there ending Feb. 20 (the show seems to be headed also for a New York engagement as of last report, but casting plans remain uncertain in view of Makarova's accident and possible return).

They say they also are in negotiation on joining a major American ballet troupe (there were rumors this was to be the New York City Ballet, and Valentina has said George Balanchine discussed with her that possibility, but company spokesmen this past week denied that any such development was in the offing). A national tour with the Kozlovs' leading a small troupe of their own also is under consideration. The Kozlovs still are in demand as guest artists, and have a number of engagements in view for 1983, with companies such as the Berlin Ballet. They also are in the planning stage with ABC-TV, they say, on a filmed special about their defection and new careers.

The Kozlovs have more than one reason for beneficent emotion about the United States, among them the fact that they met for the first time in this country on that 1973 Bolshoi tour. Much of their courting took place in Washington, D.C. In fact, Leonid says, "I still remember seeing her [Valentina] at a White House party, in such a pretty dress -- you can see where my good feeling about this country started." Two months after the end of that tour, they were married.

Do they ever find it problematical being both part of the highly competitive world of ballet and married to each other?

"Only in rehearsals," says Leonid, glowering lovingly at his partner. "She is very difficult in rehearsals."