You're not prepared for it at first. Not prepared because you've read all those stories about how she looks like she's about to burst into tears all the time, how her eyes are just two big tear ducts, how her books about despairing, passive tragic heroines are all autobiographical.

So, the first time she says something funny, speaking in that whispery voice, ending her sentences and then gasping back her giggle so as not to disappoint your image of her, you think maybe you misunderstood. But then it happns again. And again. And finally you know you 're not mistaken.

Joan Didion is a comedian.

This is a shock. Because Joan David - author of "Play It As It Lays," "Slouching Towards Bethlehem" and now "A Book of Common Prayer" - is usually pictured as grim and sorrowful, sharing with Joyce Carol Oates the distinction of being a Dierdre of modern American Literature.

Her heroines suffer abortion, and angst, bad lovers and worse lives, always wandering listlessly through sweltering climates in airlines, meaningless existences.

Her journalism, too, never avoided the dark side of the soul, even the grotesque aspects of love, of marriage. Yet here she is . . . giggling.

Joan Didion, currently considered one of the most important novelists in America, is viewing the world with a zany, offbeat, Lily Tomlin-sense of the absurd.

She doesn't look or act or even talk the way a comedian should. In fact, her delivery belies any humor at all.

She is a tiny person. That is what strikes everyone first who meets her. She is not only very short, but also very thin, with tiny bones, small slim fingers and limbs. She looks crushable. Her strawberry blond hair she pulls back to one side with a white flower, which gives the impression of her being even more delicated.

She is ladylike, elegant. She dresses in Chanel suits with high heels and she talks so softly that sometimes you have to strain to hear it. It's usually worth it. She rarely finishes a whole sentence but when she does she generally gives off a little laugh at the end.

What probably misleads people until they really get to know her is that in repose she doesn't have a happy expression. The eye-brows go down and the lines around her mouth, nose and forehead have a declining swoop to them. But that makes her remarks even more funny because they are so unexpected.

He is clearly baffled about it, and the other day, curled up on the sofa of her room at the Midison Hotel sipping tea, she tried to figure it out.

"I don't know why people say I am sad. It really puzzles me. It bothers me terribly. I don't think it has anything to do with what I write. I keep thinking it's because I'm physically small and I look too thin and ummmm. . . .if you are asked a question like where does your despair come from. . . .you answer it the way you can." And she shrugs and laughs.

"I noticed that the reviewers of "Play It as It Lays" thought it was autobiographical. It's not. Any more than "A Book of Common Prayer" is about human emotions. It's a book about history." She pauses for a second, then says "to the extent that I would ever write a book about history. I thought I had some very funny things in there. And there's a very clear eye on the heroine. It's not a subjective eye. I thought it was very funny. John did too."

Author John Gregory Dunnel is her husband and sometimes collaborator when they do screenplays. They did the original screenplay for "A Star Is Born."

(Though Didion and Dunne get credits and a percentage of "A Star Is Born" because they conceived the idea and developed the structure, they had nothing to do with it after Barbra Streisand took over and changed it considerably. The reason: "Our script really didn't have a part for her," says Didion in a burst of laughter.")

"I have the constant sense, whenever I read anything about me, that my marriage is about to break up," says Didion . I once said in LIfe magazine that I was in Hawaii in lieu of getting a divorce.Well, everything you do is in lieu of getting a divorce. In fact the act of being married is in lieu of getting a divorce," and she stops, eyebrows knitted, and thf gigles.

"John," she says, "is even more puzzled by people's image of me as always being sadder that I am. He probably perceives me as being very cheerful and undepressive."

Talk to almost anyone who has met Joan Didion and they will tell you, "She's very shy, you know." It's the one thing about her which seems to be universally agreed upon.

She, however, disagrees.

"I'm just reticent," she will say.

"My whole style is rather laconic."

She says that she makes a distinction between being shy and being reticient. "When I was about 25 I decided that it was ridiculous for a grown woman to be shy. You know what I mean? So I became 'reticent.' I think people always perceive of themselves as shy. But it occurred to me that I had been using it as a crutch because I was small. And it had a crippling effect."

One of the reasons, of course, that people see her as the tragic figure they do is because of the hapless despairing heroines and she has often been said to be preoccupied with and totally accepting of the evils and sad lot of mankind.

"I don't think," she says, "that I have any acceptance of the evil of mankind. I'm shocked by it. I think that's an absurdity. I think I'm very romantic and I'm easily shocked when my romantic idea of how things ought to be doesn't turn out. I'm a romantic egalitarian."

She also, she insists, had a very happy childhood, growing up in Sacramento with her parents, whom she gets along with very well, and her brother.

However, she does say she was "one of those children who always thought of bridge would fall in if you walked across it." Not exactly a pessimist. "I wouldn't say so, just a ummmmmm. I was just one of those fearful children, always working out how the finicular at Royal Gorge would crash. Yes, I did have a happy childhood . . . except for these terrible fears. I thought about the atomic bomb a lot . . . after there was one." She trails off into laughter.

Normally one expects "important" novelists to hibernate off somewhere or at least live somewhere near New York so they can be a part of the literary social world. Yet Didion and her husband live in California, 40 miles outside of Los Angeles on the beach at Trancas. And they are definitely part of the Hollywood world, certainly at least, as much as they want to be. They write at the beach, but will drive in to town at least three times a week to go parties, see friends, have meetings.

The people they know and see include the Hollywood establisment, Warren Beatty, Julie Christie, Jack Nicholson. Michelle Phillips, Jane Fonda, Robert Towne, Helen Reddy Norman Lear, plus studio heads, record company executives and other screenwriters.

"We live there in a removed way," she says, "People don't just drop by. We're so far out."

Though she lived in New York for eight years while she was working for Vogue she says now that it makes her nervous. "I always feel overcharged. I really love living in California. It's very quiet, very simple. It's not orgaized. It's not ummm, I feel much stantly feel the weight of other peofreer than in New York. I don't conple's social organization. It's an aloneness. You make up your own rules as you go along.

"Our best friends are writers but they don't talk about it. If you're a writer you're always afraid it isn't going to work out. So you don't want to be asked what you're doing and you don't ask. And there aren't many who aren't writers who care. So you can operate without other people's opinions. You don't have to worry about how what they're doing might conflict with 3hat you're doing, how much was your advance compared to their advance."

Didion thinks living in Hollywood as a writer is very much like living in Washington would be.

"There's a central industry in Washington that is something other than what you're doing."

She says it's not a competitive fear she has, but a fear of distraction. "I was amazed last week in New York, she says, "at the number of sheer opinions I heard. I heard. I could spend months at a time in Los Angeles with people I really enjoy and live without hearing a single opinion." She laughs delightedly at this sudden realization.

"In New York, you have to take a stand on, everything. Often, I don't know whether I like something or not. In Los Angeles there's a constant exchange of information about people, about deals, but it's peripheral to what we do so it doesn't distract."

She says it took her about a year to get used to Los Angeles after she and Dunne moved out there.

"Nobody," she says with a laugh, "even looked at me at parties. There isn't a great deal of eye contact out here. And we seemed to be meeting the same people over and over. It's like that line in 'The Last Tycoon.'

"We don't go for strangers in Hollywood.' Well, it's true," she says. "It changed for us because we were, ummmmm, we were there. After a while. There's a distrust of people who are outsiders.

"The first time I was ever really aware that people weren't talking to each other was last year at a party that Bob Towne (screenwriter for 'Shampoo') had for Barbara Howar. I was terribly aware that night that people weren't acting the way people act in other places. Barbara was being terrific, trying to make conversation and she wasn't getting anything back. I had to keep telling her it wasn't anything to do with her. And that actually it was really a wonderful party.

"There is a constant perception of imperceptible shifts of power. Another whole style is indirect. The social life out there tends to drift into ummmmm, into radical ambiguity."

She laughs at this and takes out another of the many cigarettes she smokes constantly.She ponders this whole Hollywood scene she is at least partially a part of and laughs again "I think," she says, "I probably am more comfortable with it. The social life is based on disguising opinions, not expressing opinions. I'm not," she says, "terribly comfortable with expressed opinion for myself. I don't form opinions very fast. I don't seem to have instant reactions."

"The Industry"

The people Didion and her husband see are the stars of Hollywood, the celebraties, the movie moguls, because, besides writing fiction, they make their money by working in "the industry." And they are very much a part of that world.

"I have never had a marked social gift," she says. "It has never been easy for me. I sort of fell into this style. It's a kind of closed society. It has very peculiar rules. They are very sensitive to critism. For us, it was not just a matter of being around, that we were accepted. It was that we started working in the industry. That made us as vulnerable as they were.

"Consecutive conversations," she says, making what sounds like a final summing up of Hollywood society "are not highly prized. There is a Pinteresque quality to a lot of it. I'm just temperamentally comfortable with allusions and unfinished sentences. That's why I love it there." Swarming Firelies

Part of what fascinates Didion about Hollywood, although she has never really written about it, is the fact of celebrity. "I would like to learn about celebrity," she says, "I don't understand it. I would like to understand it and write about it."

She remembers watching Robert Kennedy's funeral train on TV in Hawaii and as it turned to darkness all you saw were flashbulbs. And she says she was reminded of that night when, last week, she went to a small private party at friend Jean vanden Heuvel's in her honor and Jacke Onassis was there. Afterward, she says, coming out of the house, there was a group of photographers waiting, poping their flashbulbs until they were nearly blinded.

"Those constant flashbulbs suddenly made me think what it would belike to be famous, to have those fire-flies swarming around your eyes all the time. I would like to write a novel that dealth with it. The mystery of celebrity and who creates it."

She hasn't, she says, written a Hollywood novel, though many people thought "Play It As It Lays" was one. "Maybe I've lived there too long to address it."

They have, in fact, lived there since shortly after they were married, which was in January of 1964. He was working for Time, she for Vogue, and they quit their jobs and moved to California to free lance.

They had no money except the $300 a month she earned doing movie reviews for Vogue. Aside from their writings they began to collaborate on screenplays at which they have been very successful. "I'd never do a screenplay without John," she says. "He's the only collaborator I've ever had. And there's no ego investment in movies."

It is Dunne, she says, who edits her and Dunne who gave her the rather unusual title of "Book of Common Prayer."

"He suggested it. I thought it was a wonderful title but I didn't know what it meant for a long time.In relation to the book. And I liked the nervy aspect of it." She laughs daringly for a moment, then turns serious. "Well, it was a prayer. A prayer that I would finish the book. I was scared of the book. So the title influenced the style a little, those repetitions showed up. . . ."

What it is not is a book about a woman, she insists, nor are any of her books, though feminists are often criticizing her for creating female characters who are losers.

"I never thought these books were particularly about women. Women are the main characters because it's easier for me to write about them. But their problems are not particularly women's problems." Roles and Reviews

She feels basically uninterested in the women's movement. "I haven't actually thought a great deal about it. I've never felt part of any movement, I don't even like to use the word feminist because it implies that I'm anti-feminist."

In her marriage (she was 29, he 32, she is now 42) roles have never been a problem except occasionally when she tried to make them so. "When we married I had already written a novel. I was who I was. Though whenwe first got married I fell into that. I thought that I was not fulfilling my role as I should and that there was no chance that I was going to be a good wife in the traditional sense of what good wives did. John said he wouldn't have married me if he'd wanted that."

"Finally I thought I could not indulge myself any longer in self-pity. We were both starting to work without salaries and it took both of us to do it."

The same thing happened when they adopted a daughter named Quintna Roo after a province they visited in Mexico.

"The guilt about my not being able to fulfill the mother role lasted about two months. Ummmmmmmmm. May be six months . . . Then we had to earn a living again."

She says she and John are never copetitive with each other, though "even people who are friends wonder why. The only time we ever talk about it is when somebody brings it up. Both of us probably have a very pronounced family sense and I think we tend to perceive of the three of us as a unity. John gets furious at bad reviews of my things and I get much more upset at bad reviews of his."

She calls herself Joan Dunne. "I just started doing it in California after we were married. I'm known as Joan Danne in California. I noticed in New york saying Joan Didion struck my as peculiar. But I never even considered changing my byline."

Right now Didion is thinking about another novel and actually writing a book, nonfiction called "Fairytales," about herself. "It's kind of a revisionist theory of my own history. I don't think I know that much about myself. It's not a novel. I don't think I could write about myself in novel form."

She says she likes fiction rather than nonfiction and much better than journalism. "I was never a terrfic journalist," she says. "I, ummmmmmmmmm, I can't interview. I find myself writing down my questions and not the subject's answers. That's not a terrific way to operate." And there's a now-expected spurt of laughter.

Yesterday Didion's book was reviewed in The New York Times Book Review section by Joyce Carol Oates, another writer who has dealt with the darker side of life, with misery and despair. Didion was delighted with her review and admires Oates' writing. "It seems to me that we have more in common than is generally assumed," she says. "But people don't often compare us. I suppose that's because she writes so much and I write so little."

Joan Didion thinks that now, however, there is a real possibility that she might be able to write a happy ending. "I think I could now," she says. "I don't think I could have 10 years ago. Maybe because I hadn't seen as many happy endings.

"Though," she says, a giggle beginning to form, "I can't think of any offhand."