IN her 20 years as a theatrical costumer, Sally Ann Parsons has seen a lot more of the stars than we have.

Bette Midler without makeup, David Byrne in his underwear, the “Cats” crew without claws.

Somewhere along the way, the demure new Dame Julie Andrews gave her a kick in the head.

“That was an accident,” says Parsons, eyes twinkling. “I was hemming a dress for ‘Victor/Victoria,’ and she didn’t realize I was behind her. She’d meant to kick the train and hit me instead. She was horrified, of course – but I knew I could dine out on that story for a while.”

Actually, there’s a lot Parsons could dish about, considering she spends her time devising ways to make short stage stars seem taller and chunky ones seem slimmer, among other optical illusions.

But Parsons is discreet. “Some of the things that happen during fitting you don’t want to publish in the paper,” she says.

That’s why she won’t name the actress “with very large breasts – and proud of them” who’d insist her bust darts be exactly 4 inches long, and measured them herself.

And she’s way too busy to gossip.

The other day the crew of Parsons-Meares, the Chelsea-based costume-supply company Parsons founded with her husband, James Meares, was frantically sewing tutus for a parade in Disneyland.

Tufts of fake fur lay elsewhere, soon to be turned into gorilla suits for a “Tarzan on Ice” show.

While Parsons-Meares makes costumes for ice shows – when she isn’t dressing ballet dancers, cats and the occasional diva – Parsons’ biggest love is theater.

Growing up in California, she got bit by the bug early. Her parents were both actors, and she made her Broadway debut at age 9 in a show called “Now I Lay Me Down to Sleep” – promptly upstaging its star, Fredric March, by sitting on a shed and swinging her feet in a pair of bright red shoes.

That, if nothing else, taught her the power of accessories.

“I was severely reprimanded and my red shoes were taken from me for attracting attention and making March forget his lines,” she laughs.

Little Sally Ann didn’t realize at the time that she was violating the costumer’s golden rule: It’s not the clothes that count, but the performance.

“You don’t want to overpower someone with a costume,” she says. “You want to make their performance better or more effective, and make them look the way they’re supposed to.”

Therein lies the challenge.

Getting Julie Andrews into tailored mode for “Victor/Victoria,” wasn’t easy, Parsons recalls.

“She does not have a manly figure – she’s very curvy,” she confides. And so the costume-makers had to get around that by “pushing her in a bit, with a bust-binder.”

Another actress who needed some pushing in was Jayne Atkinson.

Shortly before she starred in “The Rainmaker,” playing the spinster to Woody Harrelson’s winning con man, the actress had had a baby – not exactly the virginal look she was going for, Parsons notes.

So she and her team drew attention away from Atkinson’s waistline by artfully placing her pockets lower down on the dress, where they’d divert the eye.

Hiding a pregnancy, either impending or just past, is less of a problem in “Phantom of the Opera,” another show Parsons creates costumes for.

“Phantom” is set in the 1880s, Parsons notes, a time of “bustles and bodices that hold everything in. Remember, hoop skirts were invented because Queen Victoria was pregnant all the time.”

When it comes to looking slimmer than you are, Parsons says, nothing beats foundation garments. Even some ballet dancers she’s dressed have been known to wear support hose in pursuit of a leaner line.

But there are other tricks to making those on stage seem slimmer than they are.

“There’s an old opera joke about the little blue dress on the front of a very large black one,” Parsons says. “It’s actually been done: dressing a large singer in a size 20 black dress and sticking a size 4 blue dress on the front of it!”

True, it’s a lot harder to get away with that one off stage. But other optical illusions, Parsons says, work just fine.

“If someone’s shoulders are too broad,” she says, “you give her a long, vertical neckline. Smaller, narrower women do better in horizontals.”

Smaller women who’d like to look taller also do well in vertical lines, she notes. When she dressed Midler for her “Bathhouse Betty” album, the Divine Miss M looked stunningly statuesque in a black-and-white striped dress.

But it’s not just the stripes that give Bette her height, Parsons confides: “She’s standing on a stool.”

“With small people in general,” Parsons says, “you don’t want to give them too much bulk because you’ll lose who they are.”

When it came time to dress Sarah Jessica Parker in “Once Upon a Mattress,” Broadway designer Jane Greenwood didn’t want to overwhelm the delicate star under a heap of clothing.

So for Parker’s nightie, the designer chose…polar fleece. It suggested a flannelly, soft and cozy look but wasn’t overly bulky.

At first, Parsons couldn’t believe it. “You’re sure you want to use polar fleece?” she asked. “But actually, it was tremendously successful.”

And while Parker may have sweated more under the lights, Parsons says, “she never complained. And she looked great!”