Milos Forman didn't think he'd be missing anything really, by declining an invitation to attend the Egyptian-Israel treaty signing. The night before his movie version of "Hair" had enjoyed a rousing benefit premiere for the American Film Institute at the Uptown. President Carter's foreign-policy advisor Zbigniew Brzezinksi had been a satisfied member of the audience; and while discoing the evening away at the party afterwards, he had invited the "Hair" crowd to the next afternoon's diplomatic show. Forman preferred to keep a noontime reservation with the Metroliner and catch the highlights of ceremonial political history on television.

But Forman was still savoring the reception to the movie. "I was very happy last night," he said. "Before I wasn't sure what to expect. I thought maybe a split between people who were part of government in the '60s and people who were protesting the war. But both types were moved. For me it confirms the contradictions. I think establishment people and anti-war people had more in common than they knew. I think they all wanted the same thing, which was peace. Some wanted peace through victory, some through defeat, some through a little of each, or an idea of something in between, but all the same goal."

After a few moments of reflection Forman remarked, "You know we had Army's cooperation in making the film?" In revising the always inadequate book of the Broadway show, Forman and screenwriter Michael Weller came up with a stirring climactic episode set at an Army base, where one of the principal characters, Claude, is in infantry training and his hippie friends from New York impulsively decide to pay him a visit. The sequences were shot at Fort Irwin, a California National Guard installation near Barstow.

"It was a struggle, "Forman continued, "but we finally got the Army to accept us, and it meant a lot. I guarantee you it would never have happened in any other country in the world. The soldiers were wonderful, very easy to direct; and get true things, things that would never be the same in staged settings.

"Like our MP who keeps Treat Williams from just driving onto the base to see John Savage. They play such a great scene, but only one is actor. We told the MP just do your job. Now matter what Treat says, you do what you have to do. It's his real role, but with us there's a little extra incentive-so he really gets into his real role. Iths dramatic, so he's even more what he is. Before we start to shoot, I can help by keeping away distractions. We make the take simple and put him at ease. Then in the cutting we punctuate, tighten a little bit. But the scene is always more real and fresh because he's there.

"Your bit parts can be so important, but you have no time to audition actors for all the bit parts. The smartest thing you can do is ask for real people. In 'Hair' the MP is a real MP, the judges is a real judges, the psychiatrist is a real psychiatrist. I learned the hard way. I had to cut two small characters because I trusted casting directors, as you must, and the actors they close for me were dreadful. But who's going to test someone for a one-line part? You don't have the time. Better to get a non-actor who is playing his real-life role."

Forman's best find among his professional cast members is the young singer Cheryl Barnes, who electrifies audiences at "Hair" with her powerful solo number, "Easy To Be Hard." According to Forman, she was working as a maid when she auditioned-and transfixed him with the first notes of her mouth. She's been singing professionally for several years, but "Hair" may make it possible for her to support herself performing for the first time.

"She was uncanny," Forman said. "Perfect the first time through during the recording session, then a perfect match of acting to singing when we finally shot her number in Washington Square. She had many little parts and chorus jobs, but none that paid anything. So she's always worked as maid or waitress or something else to make her living."

"She stayed on in Barstow after we finished the locations. Can you imagine? Barstow! She got job as waitress while we shot Army sequences and just decided to stay. I think she was there for almost one year, living Barstow, waiting tables, playing a little piano, composing songs for herself. When we found her again, she's in San Francisco-living with her sister and working behind the counter at a Happy Doughnuts. Fantastic!"

Forman said he auditioned more than a dozen writers before deciding that playwright Michael Weller was the collaborator he needed to dramatize "Hair" belatedly. "Ninety percent of the writers I talked to had the same bad ideas. All psychedelic. Aiiee! They see 'Hair' taking place in a spaceship, or it's happening in the head of a hippie, everything is a fantasy. Mike was the only realist. He came out of the that counterculture era and could see the funny contradictions.

"I think maybe it helped us a little when 'Hair' was revived on the stage last year and had a flop. There were some people who thought 'Hair was this sacred thing. The flop shook a little this confidence. I loved it at first sight. I was at the very first public preview in 1967 and went backstage to meet the authors. At that time I wanted to take it to Prague, to adapt if for the theater there. After I got to know the authors, the movie version kept coming up. One time I thought we were ready to go, but the authors were very deep in astrology. They would make no decision without consulting their guru. 'No good', said the guru. So once again 'Hair' slipped away."

Forman, who was born in Caslov, czechoslovakia, in 1932, made brief trips to the United States in the mid-'60s to introduce his films, "The Firemen's Ball" and "Loves of a blonde," at the New York Film Festival. In 1967 he returned with an American filmmaking offer from Charles Bluhdorn, the chief executive of Gulf & Western, which had just acquired Paramount. Exposing himself to 'Hair' was an integral and eventually crucial part of Forman's immersion-in-America program. Forman became an American citizen last year while the film version of "Hair" was still in production.

Forman, who was residing in Paris and trying to develop his first American script, returned to Czechoslovakia for only a brief period after the fall of the Dubcek regime in 1968. The political situation reinforced rather than provoked his desire to begin a career in the United States. "It's the dream of every Middle European filmmaker to come to Hollywood and have a big success," Forman testifies. His dream came true with "One Flew Over the Cuckoo's Nest at the end of 1975.

Lighting up his first Cuban cigar of the day and puffing contentedly, Forman remarked, "This is one of the advantages of success. People suddenly want to give you things like this."

What are the advantages beyond a good smoke? "The security and freedom that comes with having money," he replied without hesitation. "And, believe me, it's the same everywhere in the world. I've had the opportunity to live under several different social systems."

Forman said he found it no harder or easier to deal with capitalist investors than state bureaucrats when trying to launch a film. "It all depends on the caliber of the people you find in authority. If the authority is in sympathy with what you're doing, you won't be frustrated working for a state filmmaking monopoly, like several of us did very productively in Czechoslovakia. The difficulty is that your future depends on the good will and judgment of that one person. If he's got it in for you, you can expect constant frustration. Now the same situation could arise here, of course, but the big difference is that you can shop around, talk to another studio, look for another backer.

Forman hopes to bring his twin sons, now 14, to the United States on three-year visas sometime later this year. After several years of applying and being turned down, he was finally granted a Czech visa earlier this year and plans to make his first visit home in late April. His boys were allowed to visit the United States soon after Forman won his Academy Award for "One Flew Over the Cuckoo's Nest." Their burning desire was to see "Jaws," which Forman confesses he never properly appreciated until he watched it with them.

They also were consumed by a passion for pinball and every other variety of mechanical game stimulating eye-hand coordination. Forman intends to install a plentiful supply of such games in anticipation of their arrival. For one thing he also finds them relaxing.

"When I'm working on a film, I can't do anything else that requires deep concentration. If I try, I just start brooding about the film all over again and never unwind. The only way to get away from such preoccupation is to play games. So I play chess, pool, pinball, tennis, cards; solitaire works well, too. The important thing is to keep your mind off the work long enough to return to it refreshed."