"The first $100 million or so is mostly Steve's" says George Lucas. "But if this film is a huge success, I'll probably do all right."

It's easy for executive producer Lucas to be so cavalier about "Raiders of the Lost Ark," the new film directed by his friend Steven Spielberg. Today the movie opens nationwide, and already just about everyone has called it a masterpiece and dusted off a slot for it in the top grossing films of all time -- four of which are already held by these two men (Lucas' "Star Wars" and "The Empire Strikes Back" and Spielberg's "Jaws" and "Close Encounters of the Third Kind").

All the attention is for what Spielberg nonchalantly calls "a B-plus movie." "We started with an archeologist/adventurer/playboy," he says, "showed him the treasure at the end of the spiritual rainbow, and then every several minutes put him in a life-and-death situation hanging off the edge of the old serial cliff."

The inspiration for this tale of Indiana Jones came from the old serials of the '30s and '40s -- episodic thrillers like "Spy Smasher," "Tailspin Tommy" and "Commander Cody." Both Lucas, 37, and Spielberg, 33, saw them only in reruns. Lucas' family didn't have a television for a long time, so he'd go to a neighbor's house and watch the 6 o'clock show: a serial and a "Crusader Rabbit" cartoon. Spielberg haunted the Riva Theater near Phoenix every Saturday afternoon, sitting through two features, 10 cartoons, some previews, a newsreel and two serials.

The deliberate, thoughtful Lucas and the hyperkinetic Spielberg decided to team up on the project the week "Star Wars" opened. "We were sitting on the Beach in Hawaii building sand castles," recalls Spielberg. "George was waiting, terrified, to hear how 'Star Wars' had done. He told me that if 'Star Wars' didn't work, he was going to make this other movie called 'Raiders of the Lost Ark.' He told me about it and I said, 'Geez, I've always wanted to make a James Bond movie like 'Dr. No' or 'From Russian With Love.' So he said, 'If I retire, it's yours.' He got a phone call, went into the hotel, came out with a big smile on his face and said, '"Stars Wars is a hit and "Raiders" is yours.'"

But "Raiders," at that point, was simply a Lucas-Phil Kaufman outline for a story about archeologist Indiana Smith -- later vetoed because it sounded too much like Nevada Smith -- the Ark of the Covenant and lots of what Lucas calls "thrills, chills and excitement." For five days, says Spielberg, he, Lucas and screenwriter Larry Kasdan holed up in Lucas' secretary's house near Los Angeles and shouted ideas at other. "It was like the line Indy uses in the movie: 'I don't know, I'm making this up as I go along.' That's exactly how the movie was developed."

Lucus remembers things differently. "The story was already laid out," he says. "It just took me five days to tell it to those guys. Because every time I started explaining a scene, they'd pipe up and say, 'No, no, no, it's not going to work. Let's do this.'"

Working under a handwritten contract Lucas had drawn up himself -- that's why Spielberg gets most of the first $100 million -- the film began on a $20-million budget and a tight 85-day schedule that nobody figured perfectionist Spielberg would meet. He made it 12 days under schedule by keeping in mind that "the difference between a B-plus movie and an A-plus movie is so subtle you'd need a microscope to see it. So we threw away the microscope and saved $10 million. We were going after quality, but we weren't going to stay up all night to get it." Advised overseer Lucas: "It's exciting; the rest will take care of itself."

The speed and efficiency came partly because Spielberg and actor Harrison Ford constantly improvised short cuts. Ford developed dysentery in the blistering 130-degree heat of Tunisia, where the cast and crew had to fan their mouths constantly to keep out flies looking for shade (one crawled into chief villain Paul Freeman's mouth during a crucial scene). Too weak to swing his whip, Indy was slated for a 3 1/2-page fight when Ford had a better idea. "We had Indy pull out his revolver and dispatch the dude," says Spielberg of the film's funniest scene. "When Harrison pulled that trigger, we were another day ahead. And every time I gained another day, my chutzpah for inventing new things increased."

The chutzpah shows, even though they left out the 3 1/2-page fight the mine train chase, the battle with the Chinese warlord and the plane crash that Indy bails out of by strapping himself between two life rafts and skiing down the Himalayas. "We decided to leave that one for Cubby Broccoli," says Spielberg. Lucas always maintained he conceived "Raiders" because it was a movie he wanted to see; now, he says, the finished product is "better than the movie I wanted to see. I was stunned to find out how good Steve really was."

So Spielberg can wash away the taste of "1941," and Lucas has another hit -- which, he says, allows him to make "not-so-popular movies." Not so popular? Next up from Lucasfilm is the third "Star Wars" installment, "Revenge of the Jedi," and two more Indiana Jones films are planned, though Lucas says they'll be "much closer to horror movies." Meanwhile, he's attempting to make a transition from celluloid to totally electronic cinema, and hoping that the Hollywood studio system which he calls "a dying animal" will rise up. But then Lucas isn't the optimist his movies might suggest: "The world isn't the greatest place," he says. "It never has been as far as mankind is concerned and whether or not mankind will wise up is extremely problematical."

Spielberg, on the other hand, thinks "we're on our way to something very glowing. So this movie isn't like trying to amuse the troops before they attack the Bulge. It's more like trying to entertain those people who are more responsible for the gross national product than we are.""

And for all those who like to say, "Boy, they don't make movies like that anymore," Spielberg has a rejoinder. "You know, 'Raiders' isn't like any of the old serials,' he laughs. "They never made movies like this before."