SO WHAT ever happened to Albert Brooks? Three years ago it looked like he was going to make it big. His short films were appearing on "Saturday Night Live." He made his motion picture debut as the pushy campaign worker in "Taxi Driver." His second album, "A Star Is Bought," received a Grammy nomination, and Time magazine called him "the smartest, most audacious comic talent since Lenny Bruce and Woody Allen." Enormous success seemed within his grasp if only he would reach for it. Instead, he dropped out of sight.

He has spent the past three years working on "Real Life," his first feature film, which Paramount is currently distributing (it is expected to arrive in Washington soon). "Real Life" is the most original American comedy in recent memory. Brooks wrote it comedy writers Harry Shearer and Monica Johnson. He raised the money for it -- didn't even read the script. He directed it and spent six months in the editing room with it, designed the print ad and created the TV and radio spots.

Brooks calls "Real Life" "a staged documentary comedy." In it he plays a comedian named Albert Brooks who joins forces with a scientific research institute and a major Hollywood studio to make a film about a year in the lives of a typical American family. (Remember the Louds?) Wall cameras sensitive to body heat and portable devices worn over the heads of the film crew will capture every moment of activity.

The Yeagers of Phoenix, Ariz, are chosen: veterinarian Warren, his wife Jeanette and their two children. Unsurprisingly, their lives immediately begin to fall apart under the scrutiny. Their first dinner sets the mood, with Warren and Jeanette arguing about her menstrual cramps while cameramen diligently circle the table.

Things get worse. Jeanette visits her gynecologist, whom Albert recognizes as a baby broker exposed on "60 Minutes Warren loses a patient -- a horse. Jeanette's grandmother dies and Warren talks about the dead horse during her funeral service. Finally, an article about the family appears in a local newspaper and they are besieged by TV cameramen whenever they leave the house.

The Yeagers are victims, not villians. Their irrational desire for celebrity -- and Brooks's -- is the result of society's celebration of it as the only goal worth attaining. "Real Life" operates on so many levels and takes on so many subjects, with such attention to detail, that it demands to be seen more than once. Brooks's cynicism is aimed at our affectations, not our aspirations, and he trusts his audience to join him in acknowledging -- and enjoying -- the utter silliness of it all.

"Albert is a national treasure," says Charles Grodin, who plays Warren Yeager in the film. "I'm delighted that we're alive at the same time. I'd like to see him have everything. He's so damn good, you just have to feel that way."

When I call Albert Brooks to set up a meeting for the following day, he suggests getting together immediately. Unfortunately my tape recorder has a dead battery, and I don't want to sit down with him without it.

"Maybe I should just jot down some of the things I might say," he says. "Okay, I'll tell you what. I'll bring a tape recorder. I'll bring batteries, I'll even bring cassettes. What size shirt do you wear?" Twenty minutes later he walks into El Padrino Room of the Beverly Wilshire Hotel with a recorder and a cassette of Emmy Lou Harris's "Elite Hotel." "It's the only one I could find," he says. "You've got 40 minutes."

We begin by discussing the genesis of some of his early routines, including the out-of-material bit. "It was time to do another Carson show," he says, "and I really didn't have anything to do. So I thought, this is interesting, maybe I can get something out of this. Most of my bits come from what's really there. You turn it into entertainment by making it a little more interesting."

He points to a painting of a horse hanging behind our table. "Sometimes I like to make up names for the horses of famous people," he says. "Like if Burt Bachrach had a horse, what would he call it? Maybe. 'Where's Angie?' If we stop now you get the rest of Emmy Lou's album, you know."

The waiter brings my drink, and a chef's salad and iced coffee for Albert, who says that he might be going to Hawall for a vacation in a few days. "Maybe I shouldn't see you again before you go," I say. "Then I'll have to go to Hawaii to finish this piece."

"Will your editors pay for it?" he asks. "Because if they will, here's what we'll do. When you get to Hawaii, there'll be a message waiting for you saying I've gone on to Japan. Then we'll go to China, and..." He stops himself. "What am I talking about?" he practically moans. "I'll never leave. I've been talking about a vacation for five years. I just never leave. It's sick, it's not healthy" He suddenly brightens. "You know what I've always wanted to do? I've always wanted to put a lung in a suitcase and send it through an airport security check. In effect, the guard would be lookin at an X-ray of a lung."

Aside from Albert's comic instinct, the most striking thing about him is his confidence in it. His jokes are delivered as casually as they occur to him. It's clear that if he thinks something is funny, he goes with it -- getting a laugh is a pleasant but nonessential bonus.

"I'll leave the tip," Albert says loudly when the check arrives. "Not really. That was just for the tape recorder."

Two days later I arrive at Albert's Hollywood office intending to observe an average day in his real life, but he has other plans: a trip to Magic Mountain to ride Colossus, this year's World's Largest Roller Coaster.

Albert calls Magic Mountain: "Hullo, uh. I'm not going to be coming up there, but if I were, what time does Colossus open." And how long is the wall "Thank you" He hangs up and laughs. "She said. It opens at 3 and there's a two-hour want Let everybody go on and then it'll clear out and you'll go later on in the evening She's planning our evening" 'You'll have dinner here, you'll buy bumper stuckers. we got a hotel room for you.... Let's go."

An hour later we join the line about a quarter mile from the ride. "The frightening thing would be if they said we could never leave here." Albert says. "Aside from all the things you'd never be able to do again, you'd have to eat every meal here."

An hour after getting in line, we pass under the Colossus sign, and Albert begins his countdown. "Six minutes, six minutes! Four minutes!" Albert screams and waves his hands in the air as our car plunges along the tracks, but the ride is unworthy of its hype. "Weightless 11 times, they said -- I only counted four." he says as we walk down the ramp. "Three good drops, no good banks. If we'd waited two hours. I would have been disappointed."

We stop at a souvenir stand to buy buttons that proclaim "I Rode It!" "Well, we rode it." Albert says, "but only because you wanted to know what my average day was like. I do it every day See what my button says. I Rode It a Million Times!'"

Looking for a place to get a salad, we pass a gift shop with a rack of dresses near the doorway. "Who buys clothes here?" Albert wonders. "Hey, that's nice, where'd you get it? 'Magic Mountain.'"

The salad hunt proves futile. "I didn't really want one anyway," Albert says as we leave the park. "I wanted to get the button that came with it -- 'I Ate My Salad at Magic Mountain.'"

Harry Einstein (better known as Parkyakarkus, a Greek-dialeet radio comedian) finally couldn't resist the joke -- he named his fourth son Albert.

"I guess I was the class clown." he says, sitting in the living room of his rented Benefict Canyon home. "With a name like Albert Einstein, you don't hide in the back. I'd read the school bulletin to the class and I'd add activities and make stuff up.It was good, a good 10 minutes every morning."

When Harry Einstein died in 1958, 11-year-old Albert, who had grown up around Hollywood comedians, already had a reputation among them as a budding comic genius. A few years later, when Johnny Carson asked Carl Reiner to name the funniest men he knew, Mel Brooks and a high school kid named Albert Einstein were the two that he mentioned.

"Albert wanted to be a serious actor," says Rob Reiner, a close friend since high school. "He went to Carnegie Tech in Pittsburgh for its drama department, and he was talking about doing all this dramatic theater. We'd say, 'Albert, you're funny. What you do best is make people laugh.' He fought that for the longest time, and finally he started doing it and liking it." He left college after three years, took the name of Brooks ("It sounded good with Albert," he says), and returned to Los Angeles to start his career.

The traditional comedy formats became his targets. The first bit he came up with was "Danny and Dave," an inept ventriloquist act that he performed on the syndicated "Steve Allen Show" in 1968. The Dean Martin, Merv Griffin and Ed Sullivan shows followed, and other offers were coming in, but even then Albert was wary of losing control of his life.

"If I'd wanted to be a big star, I could have done the dummy bit 40 times, and everyone in the country would have known me," he says. "But I didn't want to be known as the guy with the dummy, so I forced myself to keep coming up with new stuff."

He did his first "Tonight" show in mid-1972, and quickly became a Carson favorite. Albert performed as if the audience were sitting in his living room. So sure was he of his instincts that he didn't even audition his new material with friends "I tried out all my stuff on national television." he says. After doing two years to TV.I felt confident enough to put together a live act."

Albert spent three years on the road, headlining in small clubs and opening for rock stars such as Neil Diamond in large halls. The anxiety and boredom created by doing the same material night after night finally got to him during a tour to promote his first album. "Comedy Minus One," and a gig in Boston was literally the end of the road. "I was just real tired," he says. "I remember doing an interview with a disc jockey who said to me. 'Jonathan Winters went crazy; you think that's ever gonna happen to you?' I said, 'I think it's happening right now.'" In the middle of his one-week engagement he flew back to Los Angeles.

Around this time he began going out with Linda Ronstadt, a relationship that lasted two years. "I was going with Linda just before big things started happening for her." he says. "We lived together for almost a year. We liked each other because at that time we had the exact same fear of performing -- whatever that fear was, we shared it."

(Albert is reluctant to discuss his personal life, but Penelope Spheeris, who produced "Real Life," says, "Albert's women are usually real serious. His love affairs are always like 'The Tempest.'")

By the end of 1975, his films were appearing regularly on "Saturday Night Live," ostensibly the ideal vehicle to catapult him to stardom. Unfortunately, that relationship was not a smooth one.

"Albert, to put it in its mildest form, is sometimes intolerant of other people's problems," says producer Lorne Michaels. "I had asked him for three-to-five-minute films; he got me up to five-to-seven minutes, and eventually they came in at 10. And you couldn't say they were too long, because he would say, 'They're brilliant.'"

Well, they were. "The Impossible Truth" featured an interview with a blind cab driver: "Damn right, I still drive. What should I do, sit home and collect welfare?" Another film had Albert fulfilling a lifelong dream -- performing heart surgery. ("I pray it doesn't hurt, I pray it doesn't hurt," says the patient as Albert, who has forgotten he anesthesia, prepares to make the first incision.)

But the best of the lot was "Super Season," an elaborately filmed parody of network promotion spots previewing scenes from three "new" shows: "Black Vet;; (a black Vietnan veteran takes up practice as a veterinarian in a small Southern town); "Medical Season" ("But it's unnecessary. This man does not need surgery," a doctor says as a patient is wheeled into the operating room. Replies his colleague: "It's too late. He's already paid for it and we've already spent the money"); and "The Three of Us," a sit-com about a man living with two women -- a premise that apparently was not too ridiculous for ABC, which built a real series around it two years later.

When the six-film contract expired, neither party was inclined to renew. "Viewer mail rated my films the least popular part of the show," says Albert. "The Muppets were the audience favorites."

Instead of becoming a superstar, he went to work on "Real Life."

"You rode the ride, now hear the commercial," Albert says, as an ad for Colossus comes on the radio of his Honda Civic. A Mercedes with a "Runners Make Better Lovers" bumper sticker moves in front of us as we drive to a Japanese restaurant.

"Wouldn't it be great if cars came equipped with screens like that thing they have in Times Square that spells out the news?" he asks. "You could punch out your own instant messages: 'Will the Small Red Car With the Ugly Driver Please Stay a Little Further Behind?'"

What about his plans for the future? "I don't know what I'm going to do next," Albert says. "I haven't started writing another film yet. I want to see what the climate is like for 'Real Life' before I decide.

"It only makes me anxious when I think ahead. I mean, some things you have to plan, but if you think far enough ahead, you're dead. Hey, that sounds like a slogan. Let's put it on the bumper." CAPTION: Picture, A clown during an average day.