You sort of expect him to say "zucchini" or something as he sits there drinking beer under a shade tree. But as happens with most actors who aren't the characters they play, Dennis Christopher barely speaks Italian, doesn't race bikes, doesn't serenade soroity girls and basically doesn't have much in common with the wonderful nut of "Breaking Away."

First off, he wears skinny tight blue jeans, black pointy cowboy boots and dark horn-rimmed glasses. He looks like a 25-year-old, terribly intense Hollywood actor -- which he is -- and sounds like one who is having his first wrestle with the wonders and horrors of fame, which he also is.

"A lot of people get offended when they realize I'm not the person they fell in love with in the movie," he says. "People think I'm a bicycle racer, but I'm not. I hate to say it, but you fake it -- faking it to the point of perfection, where it looks natural."

It looked natural enough yesterday, Earth Day '30, a gorgeous carnival of bands, bicycles, T-shirts and azaleas, all in the delicious colors of a Washington spring. Christopher was here to promote bike riding and the earth in general at a morning rally in Lafayette Park. Then he walked across the street to meet Walker Mondale, and then he walked up the Mall a ways for another rally across from the National Gallery. It was here that he sat down with a beer and some cheese under a tree and talked about what it's like to be famous, suddenly.

"In the past year, I've been used a lot you know what I mean? But then this thing here came up, and I was just glad that I was being used for something I believed in, you know what I mean?"

Anyway, he's got the same problems as Jane Fonda and Robert Redford, who back causes and are then ridiculed because: 1) they are said to know nothing about the particular cause; or 2) is they know something, it's allegedly just enough to help along their careers.

"A lot of people resent you because they think you're doing it for yourself," Christopher said, "And maybe that is what happens. But you sure don't start out that way."

He started out in South Philadelphia as the son of an insurance salesman. Oddly enough, he really is Italian. True, it's only third generation, but that's enough to have satisfied the "Breaking Away" character who thought he had Rome in his soul. Christopher, at one time, kind of thought he did too. The story goes like this:

"I was in California doing B-movies," Christopher begins, "and I had this European fixation. I was in love with Fellini's movies. So then I got a tax return back, and with $80 in my pocket, I went to Rome with my girlfriend.

"Well, we're sitting there drinking wine and eating in an Italian restaurant and I'm just totally intoxicated and all of a sudden I look up and there's Veruschka. So I just got up and followed her, but lost her, and ended up at this piazza. Finally my friend Jeannie caught up with me, and she was just furious for me leaving her in the dust.

"But she said, 'By the way, there's your idol Fellini over there -- what are you going to do about it?' So there he was, 100 feet from me, and I walked straight toward him, totally wrecking his scene from 'Roma.' I mean, they had to put the whores back in the windows and sew up all the pigs."

This story eventually winds up with a nice ending, which is that Fellini came over to the brash young American who fawned all over him, and eventually Christopher landed a three-week job as an extra in a Fellini film, plus one teriffic story to tell at parties and anywhere else.

Somewhere between the Fellini triumph and life in South Philly, Christopher got involved in politics. It started with a civics paper in high school about pacifism, then branched to antiwar support at Temple University. sHe's long been behind the earth and environment movement, although he's kept quiet until recently because he didn't want to be just somebody "shooting off his mouth."

Being bike racer of the moment, in fact or on film, helps. He really did try to be a bike rider in fact, although after a week and a half of training he decided it was hopeless. So, in the scenes where he is grimacing and sweating while barreling down Indiana interstate, he is really grimacing and sweating while maybe half-barreling down Indiana interstate. The inserts of the speedometer at 60 m.p.h. were spliced in later.

"I took apart the movie scene by scene," Christopher recalls, "and I went to the experts and said I want to know everything involved in looking like a chairman in this one scene. I told them I didn't want to know anything about bike racing that doesn't show."

Christopher was first spotted as the possible bike racer while appearing as the epileptic brother of the bride in Robert Altman's "A Wedding." Now he's just finished shooting "Fade to Black," a movie in which he plays a psychopathic killer that may end, at least until the next movie, all the love affairs teen-age girls have been imagining with the sweet Bloomington, Ind., bike racer.

The next project is playing the blind singer in a Florida stage production of "Butterflies Are Free," opposite Farrah Fawcett.

But that's not for a while. Today he's sightseeing.