That sincere glance, that wobbly voice which rings with righteousness and soothes with compassion - these are the trademarks of actor Jimmy Stewart. He won hearts with them in "Mr. Smith Goes to Washington." He triumphed with them in "The Spirit of St. Louis." And now he's using them to sell Firestone products on TV, from coast to coast, for an undisclosed sum.

In between innings of the World Series telecast last Friday, you probably noticed another familiar face in an unfamiliar context. Danny Kaye parayed his natural sparkle and genuine charm into a commercial for Polaroid's new instant picture movie camera.

Then there is the toughest of the tough guys, the rawest of the raw, the most brutish of the brutes - cinematic bullyman Richard Widmark - whose mean image is currently up for sale to a battery company.

They used to be the holdouts, the ones who'd never do a commercial, even if it meant turning down the last strip of celluloid available for shooting on earth. There was something cheap, something almost promiscuous about hawking one's image to peddle a product, they used to say. It was regarded as hardly acting; it certainly wasn't considered very artistic; and it definitely was not acceptable in the circles one wanted to be accepted in.

But standards of art and acceptability have a way of changing with the times. And lately, stars of the stage, athletic field and political soap box have been singing a slightly different time. Sammy Davis Jr., for instance, has been singing the Alka Seltzer jingle; Ella Fitzgerald has been singing for Memorex; and Joe Frazier has been singing (sort of) for Miller Lite. Making a commercial has become a natural to stardom as signing autographs. It has become fashionable, or nearly so - and for less than you think.

"It is good press to put in that everybody is making a million bucks, but that's just not true," said Marty Ingels, a Hollywood celebrity broker who specializes in reaching the unreachable people. "We do a lot of deals for less than $100,000 with well-known names."

"There have been some million-dollar deals, and I've handled a few of them," said Stephen Carbone, a West Coast agent for a International Creative Management. "But these aren't the ones that pay the bills. Most of our deals are in the middle range."

Stars and their agents are naturally very guarded about the money they make. But Joanne Black, who handles the parade of celebrities featured in American Express commercials, says the going rate averages $75,000 per spot and up.

Even so, several top stars still refuse to be caught pushing anything except their own photographs. Among the remaining holdouts are Barbra Streisand, Robert Redford and Dustin Hoffman. But the majority are venturing into commercials. "Ten years ago, it was looked on as giving up the ghost," said Carbone. "Now it's easier to talk a celebrity into doing one. And why not? It's just another way of capitalizing on their talents."

Does a celebrity's smile or scowl really do more for a product than an ordinary face would. "There have been a lot of studies done on this," said Hal Weistein, senior vice president for Lee Burnett Co., a major Chicago-based ad agency. "But there's still a lot of argument. It's not a foregone conclusion by any moans. It depends on specific cases."

The important point is that many companies think celebrities make a difference. Many, too, have been forced, ironically enough, to turn to the stars by the rising cost of advertising - on the theory that celebrities provide more bang for the buck.

As for the celebrities, there's no mystery why they're doing more ads: It pays. For older stars, especially, who no longer get the lucrative film offers they used to, the ad circuit has become a main source of employment.

But making a commercial can mean more for a celebrity than something to help out with the bills and occupy the idle hours. It can build a reputation as much as it capitalizes on one. Consider, for instance, the remarkable success of O.J. Simpson. He was simply an All-Star, record-setting. Heinman Trophy-winning, football running back before Hertz signed him up to dash through airports pretending he is in a hurry. Now he's more frequently introduced as the "Super-Star of Rent-a-Car" than as a likely hall-of-famer.

"We've brought him up," boasted Jerry Burgdoerfer, executive vice president for marketing at Hertz.

Using celebrities does have its pitfalls and occasionally poses some interesting marketing problems. One particularly ticklish situation arose early last summer for the Miller Brewing Co. It had just completed shooting, for roughly $100,000, a commercial showing New York Yankees owner George Steinbrenner and manager Billy Martin arguing in a bar while swigging a few cans of Miller Lite. The spot ended with Steinbrenner pulling rank and telling his manager, "You're fired!"

Not long afterwards, in a moment of real-life indiscretion, Martin called his boss a liar and was forced to resign. Miller's top officers were divided over whether to run the ad. Fortunately for the company, the Yankees soon announced that Martin would be returning as manager in 1980, and so the commercial started running on July 31.

Then there is the case of a celebrity doing something controversial - as when Anita Bryant began a campaign against homosexuals. A very distraught Florida Department of Citrus, which pays Bryant to promote fresh orange juice, commissioned a bevy of surveys which determined somewhat inconclusively that the campaign seemed to be having only a minimal impact on juice sales.

Once in awhile, a featured celebrity does something plain naughty. For instance, Jane Russell, the curvaceous film actress who promotes shapely bras for International Playtex Inc., raised more than eyebrows among her sponsors when she pled guilty recently to a drunk-driving charge.

There's no sure way to avoid such embarrassments. But sponsors can minimize them by not relying on a single star to carry a campaign. American Express, for instance, constantly changes the mix of celebrities who promote the company's credit card.

All this helps the company's case, but celebrities often find cause for jitters, too. A sponsor's fortunes can take a turn for the worse as easily and unexpectedly as a celebrity's.

Jimmy Stewart was negotiating his contract with Firestone just as early reports of defective radial tires began appearing in the press. He signed anyway in June, shortly before full-scale congressional hearings were called to look into the safety of Firestone's "500" line.

Was Stewart, who had never done a commercial before, distressed by all the adverse publicity against his chosen company? He declines to comment on the matter. A spokesman for the ad agency that designed the campaign - Murray and Chaney in Hudson, Ohio - said only that Stewart is a close friend of Leonard Firestone from their Princeton college days - a fact which, the spokesman suggested, had lots to do with Stewart agreeing to do the ad in the first place.

One other worry that celebrities who do ads now have to fret about: the Federal Trade Commission. The agency recently adopted a tough endorsement policy that requires anyone who endorses a product to make a "reasonable inquiry" into the product's proported claims.

The first victim of the policy was singer Pat Boone, whom the FTC staff accused of falsely stating that a product called Acne-Statin could cure acne and was better than other remedies. Boone settled out of court several months ago, agreeing to contribute to any restitution the manufacturer of Acne-Statin might be ordered to make.

The case sent shock waves through the ad business and threatened to scare stars out of commercials altogether. But much of the initial nervousness over the FTC's action has since subsided, though some celebrities are taking special care to check out the products they speak up for.

Said Hollywood's Ingels, "I know of one case involving Hope Laing, the actress, who was negotiating to do a spot for a meat product made by the Greenwood Packing Co. She asked to taste all the company's products. Eighty-five pounds came in a giant refrigerated crate. For three days, she ate everything in it. She finally did the ad, but I was convinced when she left to do the commercial she was too fat for it."