LUNCH WITH Nanette Fabray at the Holiday Inn off I-66 in Manassas comes to $9.42, including coffee, canned music and the opportunity to observe, firsthand, a cleaning lady vacuuming the dickens out of the dining room carpet.

It almost took place in a lounge, smelling of stale beer and old smoke, because the dining room had just closed, until Fabray pointed out, with a smile that could melt an iceberg, that one of the dining room tables was so near the lounge it almost qualified as being in the lounge, didn't it? And besides, the dining room was so much cheerier.

The rules were relaxed, although not necessarily in deference to Fabray's career as a Broadway musical comedy star, as Sid Caesar's second banana, or even as Bonnie Franklin's mother on the TV hit "One Day at a Time." Fabray is currently appearing in "Cactus Flower" at the Hayloft Dinner Theater in Manassas, but whenever she drives by in her two-door silver-gray Ford Escort, people naturally assume she's on her way to or from the supermarket and invariably try to place her in their own lives.

"People come up to me on the street, and they aren't quite sure who I am," Fabray says. "I don't dress starlike and they're caught by surprise. Then they'll say, 'Oh, you're . . . uh . . . didn't we meet in Chicago?' And they'll associate me with some warm and friendly event. I like that."

At 63, she is the star as next-door neighbor--accessible, unpretentious, sunny and serene. Funny, too, although not in any show-offy way that threatens to turn the rec room on its ear. More in the bright-but-dizzy category. "George Abbott, who directed me in 'High Button Shoes,' told me, 'You are a naturally funny lady. You're not funny-looking, but you look funny. You move funny. You talk funny. Go into comedy,' " she remembers. The advice diverted her from a potential career in opera as a coloratura and set her on the musical comedy road instead.

In "Cactus Flower," which opened Tuesday, Fabray plays a prim and prickly dental assistant who blossoms into a sophisticated older woman--the role Lauren Bacall created on Broadway in 1965. "I thought it was worth a shot," says Fabray, who turned to dinner theaters a decade ago, after the death of her second husband, Hollywood screenwriter Ranald MacDougall. All his assets, she learned when he died, were tied up in a lawsuit against one of his movies. Under the community property laws of California, her assets were frozen as well.

"The court gave me a couple of thousand dollars to bury him and that was it," she says, "I couldn't touch his accounts, I couldn't touch my accounts, I couldn't take out a loan. I couldn't even sell one piece of jewelry to pay the bills. I had a young son who was going to school and I needed work. So I had to take any jobs that came along."

The experience turned her into a crusader for widowed persons' rights, "speaking out wherever I went and locking horns with the IRS." She allows herself a quiet huff of indignation. "I knew very little about tax laws then, but I know a lot more now. Do you know that in some states a woman is still referred to as her husband's chattel?"

While some of her peers look on dinner theaters as a comedown, Fabray prefers to view them as "a viable part of the industry," a repository for the frothy comedies Broadway used to send out regularly on the road, when there was a road. Anyway, if you look on the bright side, which is what Fabray does, the six-week Hayloft engagement has brought her to Washington where she has already attended one state dinner with President and Mrs. Reagan, her former neighbors in Pacific Palisades, and caught at least a fleeting glimpse of the fabled cherry blossoms.

It also allows her to pursue another off-stage role as advocate for the handicapped. Fabray was one of the first entertainers to acknowledge publicly that she had a physical impairment--a progressive hearing loss that wrecked her first marriage and threatened to bring her career to a dead stop. At the time, she was accused of sympathy-mongering (at best) and unseemliness (at worst). Network officials fought her for a year, when, as a regular on the "Carol Burnett Show," she wanted to sing "Somewhere Over the Rainbow," while also delivering the lyrics in sign language.

As a board member of the National Council for the Handicapped, the President's Committee on the Employment of the Handicapped and the Better Hearing Institute, among others, Fabray has all the statistics and arguments at her graceful fingertips. But she is most persuasive when she tells her own story as a rising musical comedy star, who discovered while playing "Bloomer Girl" in Chicago that she could no longer hear the pit orchestra.

"I thought I had a cold or something, so I went to a doctor I found in the Yellow Pages, and he said, 'Well, you have a problem we can't do anything about. In about five years, you'll lose your hearing and when you lose your hearing, you'll lose your speech and you'll be deaf and dumb.' " She still winces at the medieval expression.

"It was pretty traumatic. My whole life had been easy and wonderful and I was a success at everything I'd done up to then. All of a sudden I was one of those terrible people the world looked down on. We live in a totally different era now, but then there was a great stigma attached to being less than perfect. Anyone on crutches or in a wheelchair was to be avoided. People in the movies woke up and their hair was perfect and their eyelashes were on. No one wore glasses. You'd fall down a manhole before you'd be seen in glasses. When you went to the grocery store, your hat and shoes and gloves all matched.

"If I'd known another person in the public eye who had a handicapping problem, it would have given me comfort. But I didn't. So I kept my problem to myself. My hearing kept going down. I didn't tell the nice, young man I'd married David Tebet, now a senior vice president at NBC that I was going to be 'deaf and dumb' in five years. My God, I thought, you don't share that terrible kind of information. Deafness is a threat to the hearing, as well, because it's a breakdown in communication, the one thing we do that sets us apart from the animals. I was so neurotically involved with my problem, so totally self-involved, so insecure, it destroyed our life together.

"I'd ask myself, 'Why me?' and the answer I'd come up with was 'You are not a good person. You are not worthwhile.' It took me years of analysis, when analysis was just beginning, to realize that I was okay."

Four delicate operations on her inner ear have since restored Fabray's hearing, but she has never forgotten the anguish. "It opened my mind and my soul. What was I then? An adorable, darling, pretty little thing. I'm still not as quick or as smart as I would like to be, but I think I've been through the fire. I would have been much less of a person than I am today without this . . . this . . . phoenix-like experience."

Fabray was born and raised in San Diego. Her father was a fireman on the Southern Pacific Railroad; her mother, a stunning housewife, who assuaged her disappointment at never having gone into show business with visions of her daughter as another Shirley Temple. Fabray started out singing and dancing as "Baby Nan" in vaudeville at what she calls "the absolute lowest, crawl-on level." She still had braces on her teeth when Warner Bros. put her briefly under contract, stuck her in a couple of films, and fabricated a past for her that claimed she'd been part of the "Our Gang" comedies. "I've been trying to kill that story all my life," she says.

She came east with a revue, "Meet the People," and proceeded to make her reputation on Broadway in such musicals as "By Jupiter," "High Button Shoes," "Arms and the Girl," "Love Life" and "Mr. President." But she found her greatest audience on television in "Caesar's Hour," stepping into the shoes that had been vacated by Caesar's longtime partner Imogene Coca.

"Sid was the only person I've worked with I'd call a genius," she says. "Well, maybe Fred Astaire. I worked with him in the movie 'The Bandwagon' and I think he's probably a genius, too. But Sid had so many facets to his genius. He was always at his best the first time he did something, when all his creative juices were flowing. The more he rehearsed, the more bored he'd get, and he'd want to change things. Well, you can't do that when you're working with other people, so he relied on a stand-in during rehearsals. Then on the final dress rehearsal, when we moved into the theater, he would step back into the sketch.

"One of the few times I ever fell apart on the show--and these were the days of live television--was in a scene where we were all a bunch of gypsies. I was singing this song in double talk that was supposed to be gypsy language. Sid came up behind me with a violin. But he hadn't told me he was going to have a violin that cried. I turned around and noticed water dripping out of the violin. Well, it caught me just right. I started to laugh, tears poured down my cheeks, my eyelashes fell off. Every time I tried to pull myself together to sing, Sid would start to play this crazy violin. We never finished the scene. We just stood there, laughing with the audience for a good four minutes."

Fabray says she was as surprised as the next person by Caesar's recent autobiography, "Where Have I Been?," in which he revealed the dark side of his comic genius and the pills and liquor that propelled him through years of television triumph. "We'd have a party at the end of each show, and he would sit down and finish off a bottle of something, whatever it was he was drinking. But I never saw him drunk. He always seemed warm and generous. That book was a complete revelation. It's incredible to be that close to somebody, seven days a week, and not know him at all."

Fabray's last Broadway show, a light domestic comedy with Eddie Albert called "No Hard Feelings," had an abrupt one-night run in 1973. "Clive Barnes preferred 'The Changing Room,' " she observes tartly, turning up her button nose that was "the size of a flea bite" until she had it elongated. (It is now the size of a bee sting.) For the last few years, she's been touring the dinner theater circuit in a tailor-made comedy, "The Oscar Ladies," which allows her to play five actresses up for the best supporting actress Oscar in five sketches, with a coda showing the winner.

But her deepest allegiance lies with television, "no matter how people mock it." This May, her stint on "One Day at a Time," as a "slightly ding-y, small-town mother," who presses her old-fashioned ideas on Bonnie Franklin, comes to an end. It's been a good six-year run for her.

"You have to know it all--theater techniques, movie techniques, television techniques--to do what we do in a show like that, which is taped before an audience," she says. "That television eye looks right through to the back of your head. You can't fool it. It knows what you're thinking. It knows if you're thinking, and if you believe what you're saying. Try to be funny, and it doesn't happen. You have to be absolutely sincere. I've always played comedy completely straight. Then you get the laughs.

"Of course, I would love to have another series on TV. I'd love to do another Broadway show, although Broadway is no longer what it was in my day when there were 12 musicals and 100 other shows. I'd like someone to see that I really am a good actress and put me in another movie. But if the career were to stop tomorrow, there are enough other things in my life that are worthwhile. I take my work seriously, but it's not who I am. That's the difference. I'm comfortable with myself. I like my friends. I guess because I've been up and down, I know that life can't always be at its best. But there will be another kind of good somewhere down the line. Meanwhile, you have to make things happen for yourself."

Then Fabray gets up from the table, steps gingerly over the cord of the vacuum cleaner that is now going full tilt, gives a gentle wave and heads out to the parking lot. As she pulls the Ford Escort out into the traffic, she could very well be . . . oh, you know who . . . that nice lady who lived in the corner house in the old neighborhood and used to bring by a cake every Christmas.

What was her name?