Paul Rudd described his first movie role as "a piece of cake." A rising young theatrical star who appeared on television as Brian, the exuberant chauffeur on the short-lived "Beacon Hill," and as John F. Kennedy in "Johnny, We Hardly Knew Ye," Rudd plays the ill-fated son of auto tycoon Laurence Olivier in "The Betsy," a splashy new film version of a Harold Robbins potboiler.

"It's a weak character but a challenging role," Rudd said. "I feel very confident about it. The idea of starting out in a character role intrigued me more than the so-called romatic leads I'd been offered from time to time. The working atmosphere was totally relaxed and professional, and what more can an actor ask an playing a majority of his scenes with Laurence Olivier?

"Hero-wise, I figure I'm way ahead of the game. I've worked with Olivier and I've played Jack Kennedy. Now all that's left is ted Williams."

A native Bostonian and ardent Red Sox fan, Rudd is the only son of an Irish-American laborer who worked his way up to an executive position in a dairy company, Rudd studied for the priesthood at Maryknoll Seminary in Ossining, N.Y., but left after 18 months to enter graduate school at Fordham University. "I never had a religious vocation," Rudd recalled. "It was just one of those misguided transport of idealism that every kid who grows up in an Irish Catholic environment is susceptible to. I've written what I hope is a fairly lighthearted account of my failure to hack it as a seminarian, and I think it might be turned into a decent screenplay. Just call it a stop-over. A little pitstop in the ongoing car race of life."

Rudd began his acting career relatively late, but he retains a youthful appearance that makes him perfectly credible in roles much younger than his own 37 years.

he worked for an advertising agency and a chemical company before devoting himself to acting. The turning point was a successful audition with Joseph Papp's Shakespeare Festival in 1967. He made his professional debut as "a spear carrier" in a production of "Henry IV, Part I," began studying acting with Tony Stimal, appeared in minor roles with Lincoln Center Repertory Company in 1968-69 and toured with the national company of "The Boys in the Band" in 1969. His theatrical credits now include three of the great Shakespearean roles - Henry V, Hamlet and Romeo, which he recently completed in a Broadway revival - as well as featured roles in "Streamers," "Ah, Wilderness!", "The Changing Room" and "The National Health."

Rudd, who also played in Arena Stage's production of "The Last Meeting of the Knights of the White Magnolia," is bullish on the American theater. "The future is going to be beautiful," he declared," as long as the regional theaters like Arena continue to prosper and mature. There are so many fabulous, serious the abundance and variety of acting talent in this country is gonna knock people silly."

Richard III is the next Shakespearean role he'd like to attempt, and "I think I've done Romeo for the first and last time.It's a bitch to play and for most of your career you're either too young or too old to play it anyway. I'm gonna wait on Hamlet before trying that again. I think that needs a good long wait before I'm ready for a second assault. I still haven't played the romantic comedy roles, which I envision as sheer pleasure.

"I usually work from the rear end when I'm doing Shakespeare. When I find what that last scene reveals, that becomes the starting point of discovery about my character. There's no doubt that he's the toughest, most humiliating challenge an actor faces. He's constantly making a fool of you. I imagine him up there laughing away whenever I'm studying a role or struggling with it in rehearsal.

"On the other hand, it's so revealing and rewarding. I think it's important for American actors to come up against Shakespeare. We have an exuberance, a freshness, that I don't see as often in the British actors. To the Brits it's their language and tradition, of course, and they do it facilely. We're spunkier. We may tend to grope with the diction for a while, but we'll take more chances with the roles.

"It's a shock, by the way, going from Shakespeare to mediocre stuff. Sometimes you're in the position of switching from the sublime to the ridiculous on the same day, and the effect can be devastating. There's nothing quite as awful as the taste of a bad line in an actor's mouth."

Rudd feels that the acting vocation was always lurking inside him, even if he was slow to acknowledge it. "There was this expressive, role-playing something in me that had to have an outlet. It had to or I had to resign myself to being unhappy. It was one or the other. I decided I wouldn't be a hack or a part-time. I was 27, after all, and if I was going to change the course of my life, I was determined to be a fulltime actor, to take it seriously and do good stuff."

Rudd turned down prominent roles in "Slap Shot" and Rollercoster" before signing on for "The Betsy." He said the prospect of bodily injury scared him off "Slap Shot" and that the director, George Roy Hill, "was quite annoyed with me. He had asked me to do the role Michael Ontkean eventually played, and I was keen about it. there was much more interplay between his character and the coach, the Paul Newman character, in the original script, I think the writer, Nancy Dowd, is going to be a force to contend with. I had fewer regrets when I saw how many dramatic scenes had been lost in the finished film.

"But I became very apprehensive when I realized that George wanted to get everybody out there knocking each other flat on the hockey floor. I envisioned myself with a nice ankle injury or a broken nose that would put me out of commission for a while. It didn't seem too smart, and I think a few guys did get seriously banged up.

"'Rollercoster' just seemed like a joke. When they said they wanted me to play this young psycho who goes around planting bombs on rollercoasters, I couldn't bring myself to think of it as a sober offer of employment."