Most TV commercials are staged to within a millimeter of their celluloid lives. But a new six-spot serial for the Nissan Pathfinder is rolling down a road not usually taken.

The marketers of Pathfinder, a Japanese 4-wheel drive vehicle, last summer recruited Kurt and Marty Anderson, a youngish, funny, vigorous California couple, then sent them on a rugged 7,000-mile trip from Chicago to Rio de Janeiro, Brazil.They rendezvoused periodically with camera crews who "just basically kept the cameras rolling," Marty Anderson said. There was relatively little rehearsal, acting or retakes.

From that 5 1/2 week odyssey and what Kurt called "enough footage to make 9 million commercials," Nissan and the Chiat Day ad agency distilled six 30- second bites, presented in chronological order.

The campaign aims not only to show off the vehicle's virtues but to forge a bond between the Andersons and potential customers who share the couple's outlook, tastes and spirit. Each spot except the last contains the superimposed massage, "To be continued."

John Rinek, Nissan's national truck advertising manager, said the company's goal is to increase Pathfindar's sales 40 percent from the 28,000 to 30,000 sold in 1987, its first year on the market. Pathfinder sales represented 4 percent of all sport utility vehicles, a category that includes the Jeep Cherokee, Chevy Blazer, Ford Bronco and Isuzu Trooper. The Pathfinder sells for $18,000 to $22,000.

The campaign cost $1 million to $2 million to produce, Rinek said, or $167,000 to $333,000 per commercial - not that much different from the cost of a well-produced studio commercial, which he estimated at $200,000.

What sets these spots apart from their studio cousins, however, is their relative spontaneity.

"We didn't want to be overt" in hyping the Pathfinder, Rinek said. "But it is an awfully good showcasing of the vehicle. It's kind of the ultimate test drive."

The Andersons, who expect to receive $20,000 to $25,000 plus royalties, consider their trip the experience of a lifetime. Marty, 35, is a real estate agent, while husband Kurt, 33, is an independent film producer. They have no children.

Marty, who had acted previously, was notified of the Nissan opportunity by her agent.

Kurt said he was not gung-ho about the idea at first: "They told her it was a Nissan commercial and you're supposed to dress like Banana Republic ... but I didn't have a safari outfit."

By the time they and another couple were called for the final screen test, however, Kurt said he was dying to make the trip.

They were chosen after they and the other couple were taken to a small Los Angeles park and told to behave spontaneously while a camera rolled.

"It was kind of weird," Kurt said, and it took the ad agency and its client two weeks to decide which pair better fit the desired profile.

The road to Rio was not easy. Sometimes the Andersons arose at 3 or 4 a.m. and were driving by dawn "so it would be real pretty for the camera," Kurt said.

At the State Department's suggestion, the Andersons, their vehicle and a second Pathfinder driven by a mechanic and a medic, were airlifted over much of Central America, barely missing Hurricane Gilbert.

Marty said the Pathfindar performed beautifully: "We never even added a quart of oil." The only repair was to reclamp a hose that wiggled loose after days of rugged driving. It was fixed with a screwdriver in about five minutes, she said, and transmission fluid was replaced so the journey could continue.

For fuel they used whatever was available - leaded and unleaded gasoline and alcohol.

One highlight, aired this week, was an unplanned visit to a village inhabited by a primitive Amazon tribe that the outside world discovered about 20 years ago. Rinek said the crew filmed the tribe's children inside the Pathfinder, smiling as they experienced air conditioning for the first time.

Marty said she had misgivings about intruding on such a society but that village residents seemed to enjoy the visit.

"I had a lot of second thoughts, some real problems with it," she said. "We gave them a lot of trade goods but we didn't camp with them. We didn't bring them guns or booze or anything."

The last spot, scheduled for the week of Nov. 7, will show the Andersons' arrival in Rio, where they enjoyed the dubious luxury of a taxi ride through maddening traffic.

"Our driver said the traffic lights at night are for decoration," Marty said. "I was terrified to drive there."