As Lillian Gish left the National Theatre yesterday, a batch of fans were waiting for her, some clutching film books for her to sign, some proffering pieces of paper.

"I saw all your films when they first came out," said one man somewhat tremulously. "That shows how old I am."

A few minutes later, as she walked toward the limousine waiting for her in the rain, the man was waiting. He took off his raincoat with a flourish and laid it down on a puddle so that her feet would not have to touch the wet pavement.

She walked right over the raincoat. "Sir Walter Raleigh," she said gratefully. "I've never had that happen before."

At 84, Lillian Gish looks much the same as she has for the last few decades: light-haired, pale-complected, deceptively frail and innocent. She is known less for the roles she acts than for the person she is, a woman whose career has spanned the history of movies in this country and thus a large portion of its popular culture.

She was in town yesterday to fill in for Gloria Swanson at the "Noon at the National" program, an hour of talk and questions offered for free to whoever cared to show up. She was wearing an apricot jersey dress and a great many opals, her birthstone, and answered in a strong, clear voice the questions put to her by her manager, James Fraser, who played the part of an interviewer.

"I never had any acting lessons" she said at one point. "The only lesson we ever had was speak loud and clear or they'll get another little girl."

She was 5 when she had her first part, and has since done, she thinks, 50 plays and 100 movies. Her stardom came in 1914 with "Birth of a Nation," which was her 39th film. "Birth of a Nation" is considered to be the first full-length feature film, and it changed the movie industry.

"They're afraid to play it now," she said. "Some people think it's racial, which it's not. It was supposed to play in San Francisco a few years ago and $15,000 of damage was done to the theater, so now they don't dare play it."

Despite her fragile image, her health is excellent, she said, and she still has 20-20 vision. She begins her day with at least half-an-hour of exercise on a slant board in her New York apartment. Her looks, which give the impression of the eternal innocent, well-removed from the difficulties of life, are also deceptive. She recalled that when she was asked to play Ophelia to John Gielgud's Hamlet in 1936, she was not particularly interested.

"I always played the dear little virgins," she said. "And they're so hard to play. If you're pretty, that takes care of the first five minutes, but then it's so hard to make them interesting. If you play a vampire or a sex symbol, 75 percent of your work is done before you come on stage. I used to call them [virgins] 'gaga babies.' I thought of Ophelia as another gaga baby. But Guthrie [McClintic, the director] said, 'You're going to play a lewd Ophelia.'"

She thought that was a fine idea, and played Ophelia in a yellow dress with a red stocking in one hand and another around her neck. "I rolled around the stage," she said. "Doctors would come backstage and ask me how I knew that a mind like Ophelia's would go like that when she was going mad. So I guess we did the right thing."

"If you had a child, what gift would you like to have given it?" asked Fraser at the end of the program.

"Curiosity," she said promptly. "I don't think time makes you old. Boredom makes you old. time is your friend. And have respect for this house you live in," she added, gesturing to her body.

"If you live one-third in the body, one-third in the mind and one-third in the spirit," she said, "you'll have a balanced life."

She hasn't stopped working since she ws 5, she said later, partly because she had to work. Her last movie was "A Wedding" in 1978; before that, she did "The Comedians" in 1967. In between she has been in several stage productions, most recently "A Musical Jubilee" in 1976. She has written two books been in several television dramas, and makes numerous public appearances.

"New York is full of 'Will ya do, will ya do,'" she said after the talk."I'm busy all the time."