In the Thierry Mugler show, American model Pat Cleveland appeared on the runway in a sumptuous red velvet gown. Looking very much the Wagnerian diva, she staggered onto the runway, a golden goblet clasped in her hands. Then she pretended to take a swig from the glass and fell to the ground. The audience cheered. In a minute, she was up again and sashayed off the runway. The audience cheered again.

That's the way it is in the fickle business of fashion. It is something to cheer about when a fashion passes and to cheer once more when it is renewed. Mugler hardly meant any great message in his campy fashion presentation of clothes for next fall presented to more than 2,000 buyers and press in the Cours de Louvre. But, in fact, like that red dress that barely had time to be forgotten, many ideas recently disposed of are being resurrected.

Only a year ago designers put to rest the easy casual look that was grounded largely in American sportswear. Last season they took a decidedly different route: very sophisticated, very dressy, "grown-up" styles reminiscent of the ones mothers and grandmothers of the fashion crowd had worn in the '30s, '40s and '50s. Peplums, skinny tight skirts, stiletto heels, hats and gloves. A lot of the designers showed that kind of fashion and a lot of stores put it on the racks. It hardly seemed in tune with the woman in Washington, who gets up every morning for a half-hour with Jane Fonda and stops at the Safeway on the way home from work. They were clothes for a lush life of leisure and limousines. But the designers were eager to shake women out of their blazers and trousers because they felt women wanted and stores needed such a change.

It wasn't just the European designers who went off on this toot. Seventh Avenue designers followed suit.

"We got too dressed up," explained Sonia Caproni, I. Magnin's fashion director. "Wearing our Sunday finest all the time got to be too much."

This season's clothes, Caproni explains, do not swing back to the old way of casual informality but retain the comfort and clean look of the casual clothes with a dash of the more formal extreme. "It is a period of less pretention, but elegant at the same time," says Caproni.

She cites as an example the sweaters worn again for evening but jazzed up, as at Valentino, with a back panel of sequins. Or the suit by Karl Lagerfeld for Chloe' that starts at the shoulder nicely tailored but finishes with a soft tie at the waistline. It is the suit she sees as the successor to this year's fashion pet, the peplum suit. She also says the Lagerfeld evening dresses with humorous applique's such as a rhinestone shower down the front of a black cre pe chemise straddle the line between sophisticated and casual. "Humor helps a lot these days," she adds.

"It is the end of stifling fit," says Kal Ruttenstein of Bloomingdale's. Ruttenstein credits the Japanese for the change as much as the designers who are interpreting the looks that stores like Bloomingdale's will sell. "One year ago all you saw being worn by fashionable women was Norma Kamali. Six months ago the fashion crowd was wearing skinny skirts by Azzedine Alaia and others. Now everyone is dressing Japanese," he said, referring to the mostly black and shapeless designs that characterize what the women attending these fashion shows prefer to wear.

The easing up on "uptight" clothes and a looser fit generally are not the only clear-cut messages of the current fashion season in Paris. Add to that the return in big ways of:

* Knits. They are part of the scheme to make things more casual, even for evening wear. They are always given special touches through pattern, texture or special detail. The evening sweaters at Chanel, for example, are cashmere edged in satin and teamed with an ankle-length pleated skirt.

* The color gray. Gray is the natural successor to the somber, super-sophisticated black that has dominated for a while. It is also more casual but never dull. A gray coat or coatdress may have a black velvet collar.

* Big coats. They cover everything but not without style. Big collars or contrast trim give them an unfamiliar look without being showstoppers.

Black sequins. They appear in heavy doses on many evening clothes but always in simple shapes. The black-sequined short skirt may become as essential in some crowds next fall as the black leather skirt was this fall. Black velvet and gold and silver lame' are part of the dressing up, but best when done in simple, familiar shapes.