MOVIES : High Atop Mount Goofiness : When it comes to taking an unlikely premise--say, a pregnant Schwarzenegger--and turning it into a blockbuster comedy, nobody’s earned his stripes like Ivan Reitman.

Surrounded by recumbent grips and gaffers, Arnold Schwarzenegger and Danny DeVito are perusing a bookcase’s worth of X-rated videos between takes on the set of “Junior.” Modeled after a fertility clinic, the sprawling set, crammed onto a sound stage on the Universal lot, is rigged with an antechamber for sperm donors that the movie’s cast and crew have dubbed the Masturbatorium and turned into an unofficial lounge.

“Let’s see,” DeVito says puckishly, scanning titles such as “Triple Header” and “Double the Pleasure.” “Seen it, seen it, seen it. . . .” Schwarzenegger, dressed in a pink maternity smock that bulges over a prosthetic stomach, chuckles grimly and puffs on a stubby cigar.

In a moment the actors will venture down the hall to another set, where Schwarzenegger, his character “pregnant” with an embryo implanted in his abdomen by research partner DeVito, will explore the presumably fertile comic possibilities of a man going into labor. But for now, there is only the desultory kibitzing that makes a day on a movie set seem like a hitch in the peacetime Army.

Finally, the director is ready, and the entire surreal assemblage--towering pink-clad former bodybuilder, riotously diminutive co-star and slumping union brethren--disappears in a cloud of cigar smoke.

There is, arguably, only one director likely to gather such unlikely characters around an equally unlikely premise. And while this director hasn’t earned an Oscar, his films have grossed more than $2 billion at the box office and permeated the pop-cultural consciousness to the point that, right here on the shelves of the Masturbatorium, there’s a porn homage to his most successful movie, evocatively dubbed “Sexbusters.”

“For some reason, I get attracted to difficult premises,” Ivan Reitman says with a shrug, settling into his director’s chair as Schwarzenegger, DeVito, co-star Emma Thompson and a gaggle of supporting actors take their places. “I mean, they’re somewhat gimmicky: Ghosts exist and you can zap them; Arnold and Danny are twins; a man who looks like the President takes over and does a better job. But I try as hard as I can to find a level of reality in the fantastic premises that I latch onto.”

Tellingly, Reitman rattles off this resume without mentioning any titles. He doesn’t need to. You may not have seen “Ghostbusters” (1984) or “Twins” (1988) or “Dave” (1993), but their stories and characters have had an uncanny way of burrowing into the cultural topsoil, there to sprout catch phrases, ancillary toy lines, hit singles, talk-show debates.

Thanks to those films, as well as “Meatballs” (1979), “Stripes” (1981), “Kindergarten Cop” (1990) and hits such as “National Lampoon’s Animal House” (1978) and “Beethoven” (1992), which he produced, the 48-year-old Reitman, a Czech refugee and Canadian expatriate, enjoys a level of discretion available to few directors.

Last March, he moved his Northern Lights production company into an imposing compound on the Universal lot within hailing distance of fellow zillionaire Steven Spielberg’s Amblin Entertainment. Rather than fret over the availability of his favorite film editor and sometime associate producer, Sheldon Kahn, he simply put him on the Northern Lights payroll. And it was Reitman who helped pioneer the practice of having stars trade huge upfront salaries for a percentage of a film’s box-office take.

“I think he’s one of the few people capable of being a really excellent movie director one day and running a studio the next,” says producer Sean Daniels, who as a young Universal Pictures executive worked with Reitman developing “Animal House.” “He understands this bizarre combination of entertainment and business.”

Reitman’s financial acumen has, to his frustration, tended to obscure his filmmaker credentials.

“I am not confused at all about this one,” producer Julia Phillips sniped about Reitman in her scorched-earth Hollywood memoir, “You’ll Never Eat Lunch in This Town Again.” “He is a businessman, not an artist.”

“Comments like that really hurt a lot,” Reitman admits. “I think I am a businessman, in terms of I’m not a fool with regard to how the business works. But setting all that aside, when I start on a film, from the moment I start working on a screenplay to the last point of editing, it’s really an intensive creative endeavor. The films I’ve made are pretty complicated and have worked for a number of creative reasons, and I’m very proud of them. Since ‘Twins’ and ‘Dave,’ there’s been a kind of grudging respect.”

That tide may be turning, even if “Junior” has more in common with “Twins” than the more cerebral “Dave.” “I think after ‘Dave’ people took him a lot more seriously,” says one of Reitman’s contemporaries. “That was a huge step forward, both creatively and as an artist.”

While no one would confuse “Meatballs” or even “Dave” with a Merchant Ivory production, Reitman does bring a gloss of sophistication to the sort of mainstream populist comedies that can gross upward of $100 million. And in a field strewn with tortured would-be auteurs , Reitman is by all accounts precise, organized and workmanlike. A former musician, he obsesses about tone, rhythm and pacing. The upshot is Reitman’s better films click along with the sureness of a metronome.

“I like working with him because he has his act together,” Schwarzenegger says. “He’s well prepared. He gives you the feeling: OK, he’s not frantic, not worried, everything is mapped out, he knows what he wants to do. That’s a big advantage, unlike some other directors.”

“He’s sharp as a needle,” adds Thompson, who cut her teeth in rough-and-tumble British sketch comedies before perfecting early 20th-Century ennui in “Howards End” and “The Remains of the Day.” “Comedy directing is the hardest, because you tread that line between engaging the audience with story and character, and making them laugh. It’s a nightmare.”

Once known to sic packs of writers on problematic scripts--such as the misbegotten 1986 Robert Redford-Debra Winger vehicle “Legal Eagles,” Reitman’s first attempt to appeal to more mature sensibilities--Reitman has tended to work closely with one or two writers on his most recent projects. On “Dave,” for example, Reitman used only Oscar-nominated screenwriter Gary Ross (“Big”), who wrote the original script.

“I found him very sensitive to the aspirations of the material,” Ross says. “‘Dave’ is very difficult. You’re trying several things at once--a political satire with a fantastic premise that needs to be rooted in reality. (Reitman) was amazingly thoughtful and inclusive.”

Over the years Reitman has also learned to tickle, rather than bludgeon, laughs from the absurdist premises that form the backbone of most of his films.

Setting the tone for “Junior,” Reitman says, “was a very delicate thing. Because you can so easily go into farce with this.” Instead, the film’s best moments come from juxtaposing Schwarzenegger’s bodybuilder physique and Teutonic bearing with a feminine awareness of the physical and emotional swings of pregnancy. At one point he turns to a smitten Thompson, who is unaware of his condition, and pleads with wide-eyed sincerity: “Does my body disgust you?”

But Reitman’s satiric edge is countered by a sentimental streak, manifested in “Junior” with lingering, dewy close-ups of newborns and beaming parents. The combination of anarchy and treacle can be found in the director himself, who relished developing the authority-tweaking “Animal House” but insists that “I couldn’t make a film with despicable people.”

“I have villains,” Reitman says, “but my movies have the kind of people we’d all like to be. Go back to any of my films--even with a rascal like Bill Murray (in “Meatballs,” “Stripes” and the two “Ghostbusters”)--and you know when you get right down to the bottom there’s a good person there with the right heart who’s going to do the right thing at the right time.

“I could never make a movie like ‘Short Cuts’ or the (Quentin) Tarantino movies. They’re great films but have a very black outlook on life. My outlook is the opposite. I keep looking for that thread of decency that keeps us as humans special.”

Reitman pulls off a pair of headphones and regards the tableau before him: cameras, snaking cords and literally expectant characters. To his left, Aida Turturro, James Eckhouse and Pamela Reed, who plays DeVito’s ex-wife, hugely pregnant and slumped in a wheelchair, wait for Reitman’s cue to enter. DeVito, Schwarzenegger and Thompson huddle behind a wall, ready to burst in from a side entrance. The scene about to be shot is part of “Junior’s” climax, and though it will last no more than 15 seconds in the final cut, Reitman has spent much of the morning blocking the cameras and actors.

“This is basically a Marx Brothers scene,” Reitman says, grazing from a plate of watermelon chunks. The crux of it is the discovery of the male-pregnancy experiment by DeVito’s gynecology partner, played by Eckhouse.

Reitman pulls the headphones on and stares intently at a video monitor. “Action!”

DeVito, Thompson and Schwarzenegger stumble onto the set; Eckhouse and Reed, wheeled in by Turturro, enter from stage right.

“What is going on?” whines Eckhouse.

“Up, up, out of here,” DeVito huffs, shooing Reed from the wheelchair and depositing in it the grimacing Schwarzenegger.

“Hurry!” bleats Thompson.

Reitman hunches forward in his chair, nodding in time to the dialogue.

“This is our patient,” DeVito sputters at Eckhouse.

“What do you mean, ‘patient’--what’s wrong with him?”

“My best guess,” DeVito says, “is that his baby’s tangled in the large intestine.”

“His baby ?” Eckhouse gasps, as the entourage scrabbles for the door. “Oh my God . . . .”

“Cut.” Reitman pulls off the headphones.

“Jim?” he addresses Eckhouse, “that’s too big.”

The scene rolls out again and again, identical except for Eckhouse’s changing the inflections of “Oh,” “God” and “baby.”

Finally: “Cut. Print that. That’s it.”

Reitman pulls off his headphones and confers with the crew.

“Let’s do one more,” he says, turning from the monitor. “Something wonderful might happen, and I have to give us the opportunity to discover that . . . and, action .”

Nothing apparently more or less wonderful than the previous 11 takes reveals itself. “All my focus was on Jim and getting that moment right,” Reitman explains, “because that’s where the laugh is.”

DeVito plunks himself into Reitman’s chair and regards veteran director of photography Adam Greenberg. “Lighting’s good, Adam,” he needles Greenberg, an Academy Award nominee (“Terminator 2”). “You’ve got a future, Adam.”

Is DeVito enjoying the filming? “Yeah, yeah, having a good time,” he mutters. “It’s always a good time.” What does he think of Reitman? “He’s a very good director.” What distinguishes Reitman from the other directors that DeVito--a director himself--has worked with?

DeVito bristles, as if Jim Ignatowski has asked him to make change for the coffee machine on “Taxi.”

“He’s Canadian.”

Born in Czechoslovakia, Reitman was barely 5 when his father and mother, an Auschwitz survivor, smuggled the family out of the country in the hold of a tugboat. They eventually settled in Toronto. “I remember learning to speak (English) very quickly,” Reitman says, though a few Canadianisms--he pronounces been as bean --linger in his sentences.

Reitman attended McMaster University in Hamilton, outside Toronto, around the same time as Martin Short, Dave Thomas, Eugene Levy and other young comics, some of whom would go on to form the seminal SCTV troupe. Soon, an entire generation of Canadian talents--among them Dan Aykroyd, John Candy and “Saturday Night Live” producer Lorne Michaels--would set the tone for comedy in movies and television for much of the ‘70s and ‘80s.

Reitman, who would later find himself in the thick of them producing a National Lampoon stage show in New York in 1974 that would metamorphose into “Animal House,” instinctively grasped the withering irony of their humor.

“What happens with Canadians, suffering from a national inferiority complex from the day they’re born, is that they become great voyeurs of life--standing back and watching. I have this sort of odd combination: the optimism of a refugee and the odd, ironic cynicism of a Canadian who is sort of commenting on things all the time.”

While at McMaster, Reitman directed a comedy short, “The Orientation,” which was packaged with the 1969 Dustin Hoffman feature “John and Mary” in Canadian theaters. (“Frankly, it got way more laughs than ‘John and Mary’ did,” Reitman chuckles.)

Flush with the film’s warm reception, Reitman and some college chums raised enough money to produce a feature-length film, “Cannibal Girls,” which Reitman directed after graduation and sold to producer Samuel Arkoff. He then produced several low-budget films for director David Cronenberg.

“ ‘Cannibal Girls’ was a mess,” Reitman says. “I realized I knew nothing about directing. But in retrospect, I wish I’d kept doing that--I would have had three or four other films. This whole producing thing just got in the way. I learned an awful lot, but it’s not what’s fun.”

What drove Reitman back to directing was “Animal House.” “I had worked real hard on it, developing the script and getting John Belushi and all those people. I was hoping to be able to direct it myself, but it was very hard for the studio to go with me. (John) Landis had just finished ‘Kentucky Fried Movie,’ which was a big hit, and he did a wonderful job. But it was clear I was going to get no creative credit for it, and did not, except retrospectively.”

Reitman could console himself somewhat because, as a producer of the film, “probably individually, I made the most money from that movie of anybody. (Landis) was on a small deal, and John Belushi was paid $60,000 for his part. So I got none of the credit for the movie, but I got the lion’s share of the income. Actually, Universal got the lion’s share. I got a decent share.”

Eager to direct, Reitman dragooned Hamilton buddies Dan Goldberg (still with Reitman, he is one of “Junior’s” executive producers and directed the film’s second unit) and writer Len Blum and, with a script polish by Harold Ramis and a reluctant Bill Murray playing the lead, started filming the summer-camp romp “Meatballs” two days before “Animal House” opened. The film was a hit, and launched Reitman’s remarkable string of box-office success throughout the ‘80s and early ‘90s.

A month after wrapping photography on “Junior,” Reitman and editor Sheldon Kahn sit in a darkened editing room at Northern Lights, playing and replaying a 20-second snippet of the movie on a massive console.

Reitman looks exhausted. He’s taken only one week off since shooting ended in early July, and the deadline to turn the film over to the studio by early November to make its Thanksgiving release seems to sit leadenly on his shoulders. Still, he’s satisfied--perhaps a little relieved--with an early assemblage of the footage that he screened for Northern Lights employees.

“It feels good. It’s clear there’s a movie there, and the performances are very strong. There’s also a strong emotional kick to the movie, which is what I was really hoping for.” (It isn’t always thus. At an early screening of “Ghostbusters II,” Reitman had what he calls “a moment of exquisite focus” when he realized the last two reels of the movie “just died a horrible death. We went out and shot 25 minutes in four days to replace everything that happened from that moment on.”)

Reitman and Kahn dig in. On Reel 9, they grapple for an hour trying to transpose an entrance by Pamela Reed to speed up a romantic pratfall by Schwarzenegger and Thompson at a crucial juncture in the story. “This should be a huge laugh and it’s not yet,” Reitman says ruefully. “I’ve got a real problem because of the way I’ve blocked the damn scene.”

Down the hall, editor Wendy Greene Bricmont is sifting through a slate of takes from Reel 12 in which Thompson fumblingly tries to comfort Reed, who has suddenly gone into labor. (“Picture a garden,” Thompson soothes in a lilting Welsh accent, “with trees, and birds, cheep, cheep, cheep. . . .”) Here, the problem is more insidious. All of the takes are funny, but funny in different ways. Reitman must choose. “This is what makes you crazy--too much wealth, and how do you spend it?”

And so goes the rest of Reitman’s summer, cocooned in the sleek silver compound financed by some of the most profitable comedies ever made. As a rule, films that gross more than $100 million--films such as “Ghostbusters” and “Twins”--must play to a demographically immense audience. Does Reitman, then, consciously choose projects that will somehow satisfy the entertainment cravings of everyone from sullen mall rats to doting seniors?

“I’ve been very fortunate that my films have played over the broadest spectrum of age,” Reitman says. “Everybody loved ‘Twins,’ ‘Ghostbusters,’ even ‘Dave.’ (But) I don’t think of an audience. I place myself in every moment of every scene that I do. It’s the only gauge.

“I seek,” declares the highest-grossing comedy director of his generation, “only to please myself.”