SUNNING HIMSELF in the backyard of a gaudy Beverly Hills mansion, Steve Martin looked like some people's notion of a jerk. He wore white polyester slacks, white patent loafers and belt, a white shirt open at the collar to reveal a gold necklace and a pink polyester blazer with crossed tennis racquet designs all over it. He was nibbling Cracker Jacks.

"I've fufilled my dreams to the last inch as a standup comedian," he said, glancing at the beautiful couples floating in and out of the house, carrying colorful cocktails garnished with tiny Japanese umbrellas. "I've made the statement I set out to make: I've done standup comedy, you know. In my head, it's over as a form for me."

As he spoke, a tourist bus stopped in front of the house and its passengers crowded to the windows to stare at the pea-green, copper-roofed, artificial flower-bedecked edifice, the controversial Al-Fassi mansion (know locally as "the sheikh's") on Sunset Boulevard, one of the locations for Martins's first feature filem, "the Jerk (A Wild and Crazy Movie).

Dircected by Carl Reiner and co-written by Martin, who plays the title role, "The Jerk" is the story of nerdy Navin Johnson, the adopted white son of a black sharecropper family who accidently becomes a millionaire overnight by inventing "Opti Grabs," a device that keeps eyeglasses from slipping down noses. Then, almost as quickly, he squanders his fortune, purchasing, among other things, a mindlessly opulent house already stocked with hangers on.

The movie also stars Bernadette Peters as Navin's love interest (and Martin's in real life), Jackie Mason, Bill Macy and Maurice Evans. Mabel King plays Navin's mother, Sonny Terry and Brownie McGhee play the blues and "Saturday Night Live's" Bill Murray appears as a flamboyantly gay decorator who designed the house.

Although Martin starred two years ago in an Academy Award winning short, "The Absent Minded Waiter" and had a cameo in the conspicuously awful "Sgt. Peper's lonely Hearts Club Band" last year, "The Jerk" is intended to launch his acting career. He'll do another 4-to-5-week concert tour this summer, but the thrill is gone, he said. "After that, I think I'll lay off for a year of two.

"It would be absurd to think that I'm going to keep on drawing 19,000 people for the next five or 10 years," he said. "And its also tough. It's really hard, you know, the one-nighters."

Off stage and off camera, he's anything but a wild and crazy guy. Members of the film crew described him as "nervous" and "intensely introverted" during the first weeks of filming. "When he's not mugging, he's almost expressionless," said one.

"I haven't done an interview in a long time because I got tired of talking about myself," he said at the outset. Evidently still tired of the subject, he was less than expansive, but unfailingly polite. He looked thin.

"I've lost some weight, yes. My last tour was very grueling and I had to cancel two dates because I was sick" (he reportedly collapsed from exhaustion). During that tour, at least one critic rapped him for delivering a perfunctory performance, Martin said. "And he was right."

Audiences at Martin's concerts in recent months apparently came not so much to laugh as to cheer their favorite routines (his greatest hits), not waiting for punchlines, bursting into applause at the first hint of an attack of "happy feet." Other comedians have called the phenomenon "rock-'n'-roll comedy."

"It's kind of like being Jesus or Hitler," Martin said, echoing the sentiments of John Lennon at the height of Beatlemania.

Like his character is the movie, Martin has become a millionaire (though with considerably more effort). Last year alone, he did more than 60 one-nighters, commanding a minimum of $75,000 a night. He's sold 5 million albums and 1.5 million copies of a single, "King Tut." For "The Jerk" screenplay, formerly (and perhaps more aptly) titled "Easy Money," he reportedly was paid $500,000 by Universal Studios, which guaranteed his production company, Aspen Film Society, 50 percent of the movie's profits.

He recently purchased a house in Beverly Hills and "gifted" (as the Hollywood verbsmiths say) his agent with a new Rolls Royce. Meanwhile, his first book, the essentially nonsensical "Cruel Shoes," seems destined for the best seller lists with an initial printing of 200,000 copies in hardback.

Budegeted at just under $5.5 million, "The Jerk" already is booked into 700 theaters for its Christmas release. Even in the chancy world of movie making, it looks like a sure thing.

"We never pandered to this project," said David Picker, the movie's co-producer (along with Bill McKuen, Martin's manager). "We never thought 'Let's make a quick buck with a Steve Martin movie." This picture is romantic and quite touching, and that's going to surprise a lot of people because everyone's expecting an extension of Steve's (live) act."

"Steve and Bernadette are so sweet together." effused director Reiner. "It's Douglas Fairbanks and Mary Pickford."

However, it's hard to imagine Fairbanks wooing Pickford by singing (as Navin does) a sing called "Animal Lips," or even "I'm Picking Out a Thermos for You." Later, Navin and Marie (Peters) are married in a Voodoo wedding ceremony.

"We wanted the house to look like it was in bad taste, so we had out set decorators come in and . . . "I'm afraid to talk about the house because they were nice enough to let us use it," Martin said.

The mansion gained national attention last year when it it was purchased for $2.4 million incash by a 23-year-old Saudi Arabian sheikh, Mohammed Al-Fassi, and his 19-year-old wife, Sheikha Dena.

The young couple incurred the wrath of sine Beverly Hills residents when they spent an estimated $2.5 million redecorating the house to their own unique taste. Which included putting plastic flowers inplanters all around the property and painting the hair and genitals on some of the statues on the veranda. That was enough to stop traffic on Sunset Boulevard and draw crowds of tourists to the swank residential area.

Amid a blast of publicity and public outcry in the following weeks, the statues came down and Sheikha Dena returned to Saudi Arabia. Sheikh Mohammed followed, but not before he reportedly married an actress in a bar after showering her with $175,000 in clothes and jewelry.

After touring the interior of the mansion, visitors to the movie set invariably emerged slack-jawed. Inside, lime green, hot peach and ruby red rooms trimmed in gold and floral patterns offer a petrodollar version of Versailles. Velour paper, enamel murals and crystal-dripping sconces cover all walls. Chandeliers rotate electronically. There's a fully equipped disco, about a dozen bathrooms and a completely outfitted barber shop.

The Sparklettes-type cooler of Chateau Lafite Rothschilde was a prop, as was the paper cup dispenser of Waterford crystal. But the copper metalflake bathtub in the shape of an opening clamshell was not. Neither was the zebra skin revolving bed, or the sauna with riped-in music and dial-a weather condition or the life-size, free-standing macrame camel in the living room. Summed up one crew member, "You couldn't even explain this a bad acid trip."

Though Martin has enjoyed unprecedented success as a standup comedian, "Now that I have a chance to do movies, that's where I'm going.

I've had the thrill of going up fast. Now, I'm in the position where I'm so ripe, there's such a predictable backlash from the critics. One reviewer even said that I represent the demise of American comedy. But I finished (standup comedy) before they finished me. That's my policy." CAPTION: Picture, no caption