NEW YORK (AP) _ The idea and the 6:40 to New Rochelle pulled in about the same time.

Grand Central Station is an odd place for inspiration. People rush through by the thousand. Nobody seems to take a moment to ponder.

But John Strohmeyer and Charles Borghese found an idea there. They saw it in a little booth, a financial service counter for commuters who have everything - everything except time.

To Strohmeyer and Borghese, a young art-and-copywriting team at the Marschalk advertising agency, the salesman in the booth seemed to be dispensing over-the-counter money advice the way newstands sell magazines.

Fast-food finances, they thought.

And that's how a commercial was born.

To advertising agencies, commercials are mini-documentaries, 30-second pieces of art. The following is a behind-the-scenes look at one TV ad campaign -from brainchild to storyboard to casting to shooting. The process is elaborate, all for just 30 seconds sandwiched between the viewers' favorite programs.

Borghese the copywriter and Strohmeyer the art director were assigned to develop a commercial that would distinguish MONY, the Mutual Life Insurance Company of New York, from the many other financial institutions selling IRAs, CDs and Money Markets.

Filled with their rail station image of IRAs wrapped like Big Macs, they retreated to Strohmeyer's office, where a miniature basketball hoop hung on the wall.

For days, they batted around a sponge basketball and their ideas. By the end of the game, they had created a commercial storyboard known as ''Fast Food'' that parodied financial advice-to-go.

''We wanted to capture the craziness of how people do their finances,'' said Strohmeyer. ''It's not that they're ignorant or foolish; it's just that they don't know any better.''

Down the hall, art director Marty Weiss and copywriter Peter Levathes were developing another MONY ad. Their production, ''Financial Institution,'' lampooned a financial officer whose sale pitch is identical for any customer - middle-aged man, professional woman and newlywed couple.

The two 30-second commercials will debut on CBS's ''Space'' miniseries in April.

Ten years ago, Marschalk made a name for MONY with a breakthrough campaign.

A young unknown actor named John Travolta was shown walking down the street in the early morning light. He said his father had worked hard to make a better life for him. ''He had it all planned,'' Travolta said, tears in his eyes. ''There's only one thing he didn't plan - he didn't plan on dying.''

It was the first time an insurance company had mentioned death on the air. The commercial made stars of its lead actor, art director (Andrew Langer) and copywriter (Marshall Karp). Langer is now Marschalk's president; Karp is its creative director.

This time, MONY and Marschalk opted for a different approach - humor. ''I think humor helps make a commercial memorable when you don't have a huge budget,'' said Langer.

''We could have had beautiful, warm scenes, but we wanted to shake the viewer and grab him,'' said Irene Schultz, MONY vice president for communications services.

But humor can backfire. The message can get lost in the laughs, the company may not be taken seriously enough or the target audience can be insulted by too broad a parody. It also requires the right actors and director. The best joke in the world will go nowhere without the right delivery.

''Where's the beef?'' was just a silly question until a spry octogenarian named Clara Peller demanded an answer.

So the word went out: ad agency looking for toothy grins dripping insincerity (for ''Fast Food'' salesmen) and suspicious, confused look (for ''Financial Institution'' customers).

''Good acting will make these things come to life; bad acting will make them terrible,'' said Langer.

Paula Dwoskin, Marschalk's television producer, had three directors bid on the MONY assignment. The nod went to Michael Ulick, whose credits include the Lite Beer commercial with Deacon Jones reading an ode to quarterbacks, IBM's catchy ''We're Your Type'' and the Coca-Cola ad with a boffo performance by the short-order cook.

On Feb. 16, less than two weeks before the commercials were to be filmed, the Marschalk teams and Ulick decided on casting. Large boards displayed snapshots of actors. Like a giant tic-tac-toe game, faces were moved up the ladder, down the ladder and off the board until there was general agreement.

''Financial Institution'' was to be shot from the perspective of the salesman at his desk. A viewer can see only his suitsleeves (pinstriped, of course) and his hands reaching toward the customers.

Because the camera was so close to the desk, one actor couldn't play both hands - his head would have blocked the lens. So two hand models were hired, working on either side of the camera.

Carl Whidden (R) and Mart McChesney (L) won the audition. Their lean hands matched up. The night before the shoot, the working hands had manicures together. The other two got freebies.

''Fast Food'' had a different problem. Originally, the sign atop the hamburger-chain lookalike said ''Financial King.'' But one TV network rejected the ad, contending it would upset Burger King, a major sponsor.

Nobody liked the fall-back name, ''Bucket O' Bucks,'' as much. ''Bucket O' Bucks is what you get at a Financial King,'' Ulick protested.

Langer decided to play it safe. ''I was afraid that Burger King would complain, and then it would cost us a bucket o' bucks.''

The commercials were shot in consecutive days at the Silvercup Studios in Queens, across from the 59th St. Bridge. A crew of 30, with Marschalk and MONY executives in the wings, watched take after take. Often the process appeared to be going in slow motion.

''The reason we shoot so many takes is so we're certain to get a frame where the acting and the hands are perfect,'' said Langer. ''These details are the difference between truth or not.''

When the newlyweds sat together for ''Financial Institution,'' something looked wrong. The actress wasn't pregnant enough. There wasn't enough padding around her belly.

''Get a bigger baby in the oven,'' Ulick said.

Ulick worked hard to get the right expression from his actors. MONY's Ms. Schultz, in particular, worried that the older customer, played by Yiddish actor Fyvush Finkel, was too buffoonish. He played some takes in a stupor; others straighter. The completed 30-second commercial has him uncertain, but wary.

''Fyvush, act like you're in a strange dentist's chair, and you're a little intimidated by the atmosphere,'' Ulick told Finkel at one point.

And later: ''You're very uncomfortable. Your jockey shorts are getting tighter and tighter.''

After 42 takes, Finkel's day was done. ''It went very easy,'' he said, adding that he could earn upwards of $10,000 in residuals for his four hours on the set.

Meanwhile, under the desk, McChesney and Whidden, curled like pretzels, were really working for the standard hand-model rate of $294. Through 125 takes. No residuals.

It's not easy to twiddle your thumb with somebody else's.

Said Whidden: ''I can't wait to put this in my bio.''