Dorothy Loudon is a fretter.

She worries about never working again in the theater. Sometimes she worries about whole audiences rising en masse and stalking out in the middle of one of her performances.

When she landed the part of Miss Hannigan, the boozy bully who ran the orphanage in "Annie" like a boot camp for Marines, she worried that people would send her hate mail or "throw things."

She also worries about Suzanne Pleshette moving East.

"It scares the hell out of me, all those people who pooh-poohed New York and high-tailed it out to California to make money--and did," she admits. "They're all limousining it back to New York and taking the parts away. I want Suzanne Pleshette to go home. I don't have anything against her personally. Or Faye Dunaway. But I want Faye Dunaway to go back to the sound studio. And Cher. I would prefer that Elizabeth Taylor stayed in Hollywood, too."

Although Loudon is costarring opposite Katharine Hepburn in "West Side Waltz," one of the few hits of this year's Broadway season (it opens a three-week run Wednesday at the Kennedy Center Opera House), Loudon has done her share of worrying about that, as well. To begin with, she worried about auditioning for "Miss Hepburn," who is the muscle, as they say, behind Ernest Thompson's comedy about the residents of a dowdy apartment building on Riverside Drive, and consequently calls most of the shots.

"I was asked to go over to the Edison Hotel for auditions--I'm told they'd already looked at 500 actresses before me," Loudon recalls. "When I walked out on the stage, there was Miss Hepburn in the front row looking up my nose. I was so nervous I almost died. This is someone I watched for years on movie screens, and the mystique tends to build up. I mean, she is a big star. I read a short scene opposite the stage manager, then I went into the lobby of the hotel next door and sank into a chair. It smells like a bus stop there, with this overlay of cheap perfume they spray to make the odors go away, which just makes it worse. I thought, if I don't get home right away, I'll faint. By the time I'd made it home, there was a message on my answering service, saying I'd gotten the part. Then all I could think was, 'Omigod, now I won't be able to open my mouth in rehearsals.' "

Her Own Brand of Buoyancy

At 48, Loudon has a great sunshiny face--rather like a middle-aged Kewpie doll--capped with a fluff of strawberry blond hair. Her voice, trained to rise above the alcoholic din of the supper clubs that launched her career in the 1950s, vibrates with gusto. She has an infectious peal of laughter, which she can switch, mid-laugh, into a snarl of comic rage. Buoyancy is Loudon's stock in trade, but what makes her brand of buoyancy so irresistible are all the emotions quivering just under the surface: the anger, the frustration, the pangs of inadequacy, and, yes, the worry. Not surprisingly, she has always admired Jackie Gleason, another performer who anchored his comedy in the wellsprings of rage.

Loudon's first face-to-face meeting with Hepburn dispelled the fear that she would stand, mute and gaping, throughout the rehearsals of "West Side Waltz." "She really is the most professional actress," Loudon says now. "She knew all her lines. She is always on time. There is never any show of temperament. As soon as the first hello was said, we had a working relationship." So Loudon promptly transferred a big chunk of her worries onto the part she was playing.

Thompson's comedy was tailor-made for Hepburn, who, as Margaret Mary Elderdice, an elderly widow of crusty temperament, refuses to mingle with the denizens of her run-down apartment building. While the other ladies congregate in the lobby to gossip, Margaret Mary pounds out waltzes on her piano and haughtily savors her independence. If she tolerates the daily intrusions of a fiftyish spinster named Cara Varnum--the Loudon role--it is only because Cara plays the violin. Their duets are a pleasant way to while away the aimless afternoons. Cara would like nothing better than to worm her way permanently into Margaret Mary's appartment, and Loudon's sniveling, occasionally abject, attempts to curry favor provide Hepburn with a lively foil.

Making (Mute) Music

Both actresses mime their music-making, but Loudon took violin lessons just to be as accurate as possible and got so that she could eke out the scale. "The scraping was so awful my instructor was almost in tears from the sounds that came out," she says. "Finally she gave me a mute. Did you ever hear of that? A mute for a violin! That way, she said, I wouldn't break my lease when I practiced at home."

Harder, still, was coaxing a likable character from Cara, a slightly dimwitted old maid, who regularly empties Margaret Mary's bonbon dish, appreciates the New York Post because it "has style," and blithefully airs such mindless prejudices as "Don't you know that all hired people are thieves?"

"She really could be a royal pain in the fanny," Loudon says. "She's so totally out of the swim of life. It's been the hardest part I've ever played. I like to hear people laugh, but Cara has no funny lines. It's all character. Audiences laugh with Margaret Mary, but they laugh at Cara. But there's really not a mean bone in her body. She just aches to be needed. Oh, I could go out on the stage and play things so broadly that it would make everyone's head spin. But that's never been a temptation for me. I wanted to go beyond caricature. It has been a great growth experience for me."

Wherever "West Side Waltz" plays, Hepburn has proved a huge draw and her fans regularly bombard her at the curtain call with flowers. Loudon is philosophical. "I don't imagine I'll ever be a big star that people line up for. That doesn't upset me a lot. I like to think of myself as a working actress," she says. "Of course, people are coming to see Miss Hepburn. But they also see me, as a result, don't they? That's been a big dividend."

From Flops to Fans

Actually, Loudon has been accumulating fans of her own since she made her Broadway debut in 1964 in a musical flop, prophetically entitled "Nowhere to Go But Up." She played a nightclub singer named Miss Risque, and everyone pronounced it, with discouraging foresight, Miss Risk. If critics cheered her appearance in the half-dozen shows that followed, they usually drubbed the shows themselves. It wasn't until 1977 that Loudon finally broke the jinx, as the acidic Miss Hannigan in "Annie." Going up against a a chorus line of tousle-haired kids, a Christmas tree, Franklin Roosevelt, and a dog rescued from the Connecticut pound, she sang scathingly funny tributes to "Little Girls" and "Easy Street" and promptly sashayed off with that year's Tony.

For once, people started swarming backstage. "Mothers would come to my dressing room with their children," she remembers. "Say hello to Miss Hannigan," they'd tell them, and the kids, absolutely terrified, would hide behind their skirts and bawl, 'But I don't want to meet Miss Hannigan.' 'Oh, yes, you do,' the mothers would snap back. But a funny thing happened, after the cast album came out. I started getting mail from parents all over the country saying their children were doing Miss Hannigan in school assemblies. And they'd enclose snapshots of their little darlings, dressed up to look like me in a wig and a ratty housecoat. It was all, well, very strange."

At the height of her "Annie" triumph, Loudon's husband, arranger/musician Norman Parris, died of cancer. They had waited 17 years for him to get a divorce, and their marriage was barely six years old. "That changed my life totally," Loudon reflects. "I suppose I went a little crazy. I don't know that I've faced it yet. Then I went through a period when everyone was supplying me with blind dates. You know, 'Let's do something for Dorothy' when Dorothy didn't want anything done for her. I'd walk into a room and my eyes would zoom right to the single fellow who was just as angry at being there as I was. It's funny in retrospect. I suppose time takes care of some things. Not everything, though."

The Strangle World of TV

Loudon was bypassed for the role of Miss Hannigan in the soon-to-be released movie version of "Annie" (Carol Burnett got it), but she was tapped by Michael Bennett for the lead in "Ballroom," his splashy but flawed follow-up to "A Chorus Line." It only weathered a few months, but Loudon was nominated again for the Tony. Soon after, she was chosen to succeed Angela Lansbury in "Sweeney Todd." She claims it was the only role she was not required to audition for, and views it as "pure genius, the best material I've ever had to work with."

In the summer of '79, television also came forward with a series, "Dorothy," in which she played--shades of Miss Hannigan--a drama instructor in a school of obnoxious girls. "They called it, very kindly, a mini-series, so that when it bombed they could tell people there were only four episodes anyway," Loudon laughs. "They kept rewriting it up to the last moment, so that by the time you got the final script it looked like a Pulitzer Prize-winner compared to the others. But it was an absolute disaster. I only saw one episode--at a friend's house. I just sat in the corner, drank wine and cried.

"I think stage actresses have a rough time on TV, especially musical comedy actresses. You can do something small, if you think about it all the time, but otherwise, you tend to play for the balcony and your performance is bigger than life. Anyway, the minute that red light goes on, it's the end of the world for me. I can see 18 million people getting out of their chairs and heading for the kitchen. It's like instant rejection. At least in the theater, I've got a chance. 'Look, you don't like this? Wait a minute. I'll grow on you.' I figure I'll get to someone eventually."

Always Room for Frets

All the woes notwithstanding, Loudon's theatrical career is in high gear these days. She's weighing a new musical, a comedy, which would allow her to play three women, and a Carnegie Hall concert. Still, she frets.

She frets about the impact of inflation on the theater. ("Even I am paid too much!") She contemplates the post-Broadway tour of "West Side Waltz" with a sinking feeling. ("On the road, I eat too much, drink too much and stay up too late. It's like working nightclubs all over again.") She worries that audiences will take the generous foam rubber padding in her costumes for the real thing.

For Christmas, Hepburn surprised her costar by painting a watercolor portrait of Loudon from memory. The portrait--which looks a little like Hepburn herself, if truth be told--now hangs prominently in the living room of Loudon's Central Park West apartment. When Loudon thought about reciprocating, however, she was characteristically paralyzed.

"What do you give to the Washington Monument?" she asks. "God, I agonized over it for months. Then I went to this wonderful place and got an enormous basket. It cost me a fortune. I wanted to leave the price tag on it. I filled it with all kinds of good things to eat, breads, jams, chocolates. Miss Hepburn loves that kind of stuff. If I'd had the time, I would have sewn something. Smocks! Miss Hepburn likes to wear smocks. I could have made her a lovely smock. I probably should have. I'm really better at sewing than anything else. I've often thought that if I could have just gone out on the stage and sewn things, I would have made it in the theater years ago. You see, I could ask for sizes from the audience, then sit down and just start to sew away . . ."

The anguish churns up Dorothy Loudon's stomach, but in the telling, it invariably comes out comic. A latter-day alchemist, she tends a caldron of frustrations. She sighs broadly, rolls her eyes and then fishes mirth from the bubbling waters.