WELL SHOOT," said Barbara Cook, "This is going to be fun." The singer was standing in an aisle at the top of the Terrace Theater auditorium, looking across the steep rows of seats to the stage where she will be singing for two weeks beginning Tuesday. It was her first encounter with the small, pink and silver hall which should be suitable for her intimate singing style, and it was love at first sight. There was no Terrace Theater the last time she sang in Washington, in 1978. "It's hard to believe it was that long ago," she mused. "Somebody was just saying that '78 was the last time I was here, and I said, ' Are you sure?"

"Do you have to have an omnidirectional microphone?" A sound technician interrupted her reflection with a brass-tacks question. "What's that?" she responded, and he smiled: "You don't."

Then they went off into technical discussions of how she holds a mike, monitor speakers to allow the musicians to hear themselves and one another, and a key nonelectronic piece of equipment: "Do you have a three-quarter stool? Not a little stool" (stoopping and holding her hand at about knee level), "and if it's a bar stool, I'll have to climb up on it and that won't work."

Cook was making her advance visit without her music director, Wally Harper, who is in Hollywood doing dance arrangements and playing a small role as a pianist in "Best Little Whorehouse in Texas." But she seemed to have the situation in hand.

She outgrew ingenue roles by the mid-'60s, and has objections to them that are neatly summarized in a Gilbert and Sullivan-style song composed for her by Wally Harper: When I was 3, they said to me The stage would be my medium. But ingenue must pay their dues With unrelenting tedium, We must be sweet or quite discreet And dress in unappealing clothes. We may not tease our friends or hair, Or swear, or wear revealing clothes . . . An ingenue must promise to Imprison her virginity. All men above the age of 10 Are kept from her vicinity . . .

The parts for boys you play against, they bring out all the clones to do, And movie roles you'd love to play, They give to Shirley Jones to do, So many shows that had to close, The critics will attack her in, And it's so hard to live on guard And stay as sweet as saccharine. And up all the pros and cons, you end up with a minus score, And that is why the ingenue has died out like the dinosaur.

The kind of role that would bring her back to the stage, she says, is one that has "characters you care about," and she doesn't see much of it happening currently on Broadway. "Some of the shows now -- good shows -- are about structures and ideas more than they are about people you can root for and things that happen to them," she says. "I like to care about people when I go to the theater, so that's what I'd like to play."

Instead, what she sees are shows like "Sweeney Todd," which gave her mixed feelings: "I saw it twice. The first time, I didn't enjoy it, and the second time I was able to see the brilliant work in it . . . It would not be something I'd want to deal with every day. Of course, I'm speaking from the audience standpoint, and acting in a show is very different. But i'm certainly not attracted to the subject matter."

A similar attitude guides her choice of songs to perform. She and Wally Harper "both have a rather positive outlook, so that we're drawn to songs that have a positive message," she says. "In doing our albums, we were approached by people who wanted to have their songs recorded and asked what kind of songs we were looking for, and finally we had to sit down and formulate it. This is not something we were even aware of until then. There are many lovely songs that I would have trouble singing. I try to find songs that say something I want to say.

"Don McLean writes particularly beautiful melodies, I think, but some of the lyrics to his songs are rather negative. The song 'Vincent,' for instance. It's a beautiful song and we were going through it with an eye to doing it, and I realize it said things that I don't really believe . . . God knows, we have problems, but I don't think people are necessarily, intrinsically bad, or that life is going to get you if you don't watch out."

In her second career -- singing most often to relatively small, sophisticated audiences, Cook has established a reputation as one of the great song stylists of our time and accumulated hundreds of enthusiastic reviews. The cabaret and concert repertoire released her from the limitations of a single Broadway role and allowed her to be many people in a single evening -- but, above all, herself. The 1975 Carnegie Hall debut mader her "an instant cult figure," according to The New York Times. ". . . There is no other voice of such magnificence in popular music with the exception of Sarah Vaughan."

"She is the Broadway cabaret singger par excellence," said the New York Post in 1979. In the Minneapolis Tribune last year, critic Michael Anthony noted that "Cook's forte is the rendering of emotional states." Her voice, he said, "has deepened, become more expressive, since her Marian the Librarian days. It's still a theater voice, ample in size and weight, crisp in enunciation, and if the top register isn't as pure and bell-like as it once was, the gain is in emotional projection and in the uses of vibrato." Rex Reed was less technical but even more enthusiastic: "If I ever get to heaven and the angels don't all sound just like Barbara cook, God's gonna have a lot to answer for."

Plagued by illness (hypoglycemia) and seriously overweight the last time she came to town, Barbara Cook is looking well, happy and spectacularly slimmer than before. She considers herself, at this point, sort of a walking advertisement for the revolutionary and controversial Pritikin diet.

Cook's story is as dramatic as any of the shows she played in on Broadway in the 1950s -- shows that included revivals of "Show Boat," "Oklahoma," "Carousel," "The King and I," and lead roles in the original productions of "Candide" and "The Music Man." After some brilliant early successes, she was unlucky in the choice of a few short-lived shows; then, for no apparent reason, the slim ingenue with the beautiful voice fell victim to a weight increase that went completely out of control. for years, while she left the musical stage but played occasional straight dramatic roles, she did not know she was a victim of hypoglycemia. The disease is hard to detect because its symptoms -- weight increase and personality changes -- can be explained on non-medical grounds. But the voice was still as extraordinary as ever, as she showed in her 1975 Carnegie Hall debut -- a spectacular success which she repeated last year. Since then, she has been performing in cabarets and occasionally in concert halls but avoiding her first love, the musical theater.

What has been happening to her since her last visit to Washington?"Probably the main thing, certainly in the last year, is the way I've changed," she says. "My size has changed rather drastically and I feel better." Now, she is sitting at a small table in the innermost sanctum of the Kennedy Center, Roger Stevens' office, eating a good-sized meal of fish, potatoes and broccoli specially prepared for her and brought down from the Roof Terrace Restaurant. "The hypoglycemia is pretty well under control," she says. "I hate the idea of six or seven meals a day, which is what they recommend, so I don't do that; I'm just very careful about what I eat. It is a mess when you're eating out of traveling -- very hard. You can do it, but you have to go out of your way: I've been doing a whole different thing, basically the Pritikin regime, since February. It's a complete about-face for me, which was scary. For years I've been told to have a high-protein, low-carbohydrate diet and with a doctor's help I do the opposite now: low protein and high carbohydrate. It seems to make sense. The strange thing is that I haven't been working hard at losing weight -- that's not why I'm doing it in the first place -- but little by little I seem to be slowly losing. I think that one thing that hypoglycemia did was to make me see that I had to control my food intake -- nobody could do it for me -- or I was going to feel bad and it was going to be within a couple of hours. That's a very good learning process. I consider myself now as being allergic to certain things; I'm not, but I look at it that way. I don't drink wine because I know that if I do, in a couple of hours I'll feel bad, so it's stupid. It's like someone being allergic to strawberries and having a big dish of strawberries. You have to be a complete fool to keep doing something that's so obviously bad to yourself."

Since she has been feeling better and has her weight under control, Cook has been thinking of returning to the musical stage. "I want to find something special," she says, "so it will take a while, God knows. In the meantime, I have these other things I can do. I might be dissuaded, but I'd like to do a book musical, not a revue, something with a real dramatic content -- really a play with music. I think that's what I do best. The most difficult thing is to come up with the idea. The writing is certainly important, but they tell me the writing is easier -- though God knows it wouldn't be easier for me. So if anybody has ideas, I wish they'd let me know."

Meanwhile, "these other things" she can do include a two-week stint at the Terrace. The Cook fan club in Washington is large and enthusiastic. "I need a Barbara Cook fix," one Washington fan was heard moaning a few months ago, and when her return was announced the news ran through the grapevine like an electric current. It may be hard to get tickets in a hall as small as the Terrace, but the management is doing what it can. "Are we going to use the front?" Cook asked, trying out the Terrace stage and walking far out on a rounded extension that stretched far past the proscenium. "This part will be seats," she was told. "It gives us room for three more rows. They're already sold."