Christopher Reeve, virtually unknown actor, strolled off the 10 a.m. shuttle from La Guardia yesterday just ahead of Joe DiMaggio.

Joltin' Joe was spotted by most of the passengers, who pumped his hand as if it were still wielding a Louisville Slugger instead of a coffee maker. Reeve went thoroughly unnoticed, the difference between Superman and Clark Kent.

All of which may change on Dec. 15, when Reeve's 6-foot 4, blue-eyed body will be flashed on 700 movie screens around the country in the form of - you guessed it - the Man of Steel and the mild-mannered reporter.

"I hardly think of myself as Superman," said Reeve, who grew up in Princeton, N.J., lives in New York City, graduated from Cornell with a BA in English, studied piano at Juilliard and is a private airplane pilot and a commercial skipper of sailboats who just finished ferrying a 50-foot Swan sloop from Essex, Conn. to Bermuda.

"If anything, a role like this sends you in the opposite direction. When John Travolta walks into a disco, he heads directly for the table in the corner farthest from the dance floor. I think of Supie as a fantasy of ourselves; Clark is a tongue-in-cheek impression of who we are."

Two years ago, at the age of 24, Reeve was chosen to portray the planet Krypton's favorite son in a $50-million movie written by Mario Puzo and starring Marlon Brando and Gene Hackman.At the time, Reeve was playing on Broadway with Katherine Hepburn in Enid Bagnold's "A Matter of Gravity," whose tryouts had him at the National Theater here for several weeks.

Reeve's first return visit to Washington is for a press conference this morning at the Motion Picture Association of America, where he and Sen. Ted Kennedy are announcing, as "the presidential premiere," the world debut of "Superman" on Dec. 10. The Kennedy Center event will be a $1,000-a-seat for developmentally disabled children sponsored by the Joseph P. Kennedy Jr. Foundation.

According to Reeve, virtually everybody involved in the film, including Brando and Hackman, is turning up for the permiere. The $1,000 ticket includes a party at the home of Sargent Shriver, a reception at the Japanese Embassy and a dinner at the Kennedy Center the night of the screening. Some $500 tickets are available, with the reception cut out, and a few $125 seats come with only the Kennedy Center dinner.

Fans of TV's old, "Look. up in the sky, it's a bird, it's a plane . . ." will find that Superman has changed a bit since the late George Reeves' ubiquitous, deadpan flying cop.

"This Superman represents the best anyone could do," said Reeve. "He has peace of mind, which is expressed through the eyes. He's a pacifist who works by inspiration."

And he has a sense of humor.

"There's a sense of humor.

"There's a scene," he said, "where a crook is climbing up the outside of a skyscraper using rubber suction cups and breaking into offices, and he's climbing and climbing and all of a sudden he comes upon this red boot. I look down and say, 'Oh, the elevators weren't working, huh?'"

Meanwhile, Lois Lane (played in the film by Margot Kidder) has come a long way from "a nice little lady with a little hat who wanted to get married and have 2.3 children. She's got style: she's sophisticated, she's bright and witty; she wants to be the best reporter on the Daily Planet: she may or may not go home with a guy on the first day; she picks up checks and calls anybody she wants to."

And don't expect Superman to duck into phone booths for his costume changes.

"There is one scene when I leap out of a skyscraper and you see me change costumes on the way down," Reeve said. "But no phone booths. Have you looked at a phone booth lately? They're all glass. Supie'd be arrested for indecent exposure."

The whole idea of Puzo's script, Reeve said, is to explore the problem of "taking an old-fashioned hero and dropping him off in the middle of the 1970s. People say, 'Oh, we don't have any heroes anymore.' Well, what if we did? If you sit home and watch television - and movies are more and more like television - there's no sense of spectacle anymore. This character Superman is a real part of our American myth. He went into the trenches in Germany. This movie lives up to that.We take the audience to a place they'd like to escape to."

A lot of that escape, Reeve said, is watching Superman fly.

"I can hover in the air, or go twice the speed of the Concorde. I do loops and barrel rolls. A lot of this stuff actually happens in the air. We were shooting at Pinewood in London and I was so high up they had to give notice to pilots approaching Heathrow."

Reeve does all his own stunts in the movie. He had to gain 50 pounds for the filming, and worked out daily for two hours withe Dave Prowse, a former Mr. Universe who acted the part of Darth Cader in 'Star Wars." Some of the flying maneuvers are so complicated, he said, that it took three months to shoot the complete sequence.

"There's never been a film shot more out of sequence than this picture," he said. "Which makes it difficult to sustain the same persona for the characters. We were shooting for 18 months. I'd have been a lot happier if it had been 10 weeks. It's astounding when I think that in some shots it took 500 people to make me fly."

Half of the "Superman" sequel has already been shot, and the film's producers are awaiting box-office response to decide on the completion of part two.

Reeve himself is hoping that "Superman" will become enough of a success so that he can join his buddies on "Saturday Night Live" with a bit he's already planned:

"The planes are stacked up at Kennedy. No. 1 for departure is a China Airlines 707, No. 2 a Pan Am 757. Then you hear the tower clearing No. 3 for departure, and you see the right foot - a red boot - starting to pound against the runway . . ."