Are the stars out tonight? Tonight and every night, now that close to 100 celebrities are plugging products in commercials and advertisements.

Robert Young, Joe Namath, Bill Cosby, Danny Kaye, Cheryl Ladd, Angie Dickinson, James Garner, Karl Malden, Carol Lawrence, Florence Henderson, James Stewart and even Sir Laurence Olivier are out touting cars and coffee, puddings and Polaroids, Some, such as Farrah Fawcett and Reggie Jackson, have products named after them.

Using a celebrity to convince consumers that your widget is better than your competitor's is nothing new. Coca-Cola had stars on its trays and posters before the turn of the century, and famous film folk graced cigarettes, soft-drink and cosmetic ads from the '20s through the '50s.

But the barrage of big-name talent has intensified recently, thanks to clients' willingness to pay big fees ($500,000 or more), stars' willingness to stop turning their noses up at selling, and the need for ads to make an impact in 30 seconds or less. A sign of the celebrity boom was a panel discussion on the subject, called "Celebrities Ring Cash Registers," at the recent Advertising Age Advertising Week convention in Chicago.

Participating were Arthur Godfrey, veteran radio and TV personality (Chrysler Corp., Lipton tea and soup and a new campaign for the DC-10); actor E. G. Marshall (Maalox, North American Rockwell Corp. and political spots for Democratic candidates), and actor Eli Wallach (bank ads in New York and Cleveland). The panel was hosted by Peter Kelley, head of the commercial department of the William Morris Agency, one of the country's largest talent agencies.

Finding the right celebrity for your ads "is like putting the right person in a part in a play," Kelly said. "It's unfortunate to see a concept with no sense, just putting the name up there to get attention. The question to ask is, "What is the importance of the (celebrity), in this point in his career, to the demographic group I want to reach?'"

And it's important to remember that celebrity appearance in advertising "are not necessarily endorsements," Kelley said. "While we're very careful to make sure our clients know what they're getting into, the actor is not a super-expert in an area. They're actors, playing a part."

The actor's involvement in the ad varies. Godfrey said that he usually rewrites the ad copy in language he feels comfortable with and that he will not involve himself with a product "that I don't believe in. I won't say anything I believe not to be so." Marshall also reviews his ads' copy.

Then there's the other end of the involvement spectrum, recalled by Wallach, who told of a tipsy actor appearing years ago in a live TV commercial for the Nash. Gesturing to the car behind him, "The guy said, 'It's a piece of tin. If you want a car, get a Rolls," Wallach said.

Wallach had "never heard the word 'demographics'" before New York's Emigrant Savings Bank approached him to be its spokesman, explaining that its research showed that customers considered him a typical New Yorker. He reminded them that "my career has been spent as a Mexican bandit in Italian westerns, robbing banks. The campaign has been so effective, there were 11 robberies yesterday."

Marshall said he carefully considers the company before agreeing to do a commercial. "I would never, unless financially strapped, do Preparation H because I still want to be on the movie screen," he said. "But it hasn't hurt me to be a spokesman for Maalox."

Marshall's Maalox spots are set in a film studio to "reinforce that I'm an actor," rather than the lawyer he portrayed on "The Defenders" or the doctor he played on "The Bold Ones," he said.

There have been several campaigns using stars in what Kelley called "a shaded area," playing on their familiar roles to enhance a product's claims (such as Young, formerly Dr. Marcus Welby, advising friends that Sanka won't make them nervous. "I don't think Young could ever get a commercial past the networks advertising Eli Lilly (the drug manufacturer)," Kelly said.

The veteran product pitcher of the trio, Godfrey, had some words of wisdom accumulated during his 40-plus years in the media. "The secret of selling goes back to the days when I was in my first or second year in radio in Washington," he said. "One day I had a piece of copy to read about filmy, clingy underwear for milady. This was 45 years ago, when you didn't think sexy thoughts, let alone say them on the air.

"I read the copy, which was very prurient, so I thought someone was playing a trick on me," Godfrey said. "I tore it up in front of the mike and said, "I'm very sorry, friends. I hadn't seen this before and I'll never read it again.'"

The next morning, the station's sales manager sent for Godfrey, asking him to read a letter. It was from the underwear company, apologizing "for embarrassing me. But it also said that by 10 o'clock, 100 women had come into the stores asking for the filmy, clingy underdrawers "that made Mr. Godfrey's face red.' I learned to shock the listener into helping him remember the name of the product.