O.C. THEATER : ‘Code’ Star Loves Role,Dislikes Character : Stage: Playing a brilliant but eccentric British cryptographer in the SCR biographical drama has given the star a challenge and insight on a thoroughly disagreeable person.

Jonathan McMurtry believes that if he and Alan Turing--the British mathematician he plays in “Breaking the Code” at South Coast Repertory--had ever met at a cocktail party, the encounter would have been unpleasant for both of them.

“I wouldn’t have liked him personally,” McMurtry says. “I would have found him too self-centered. He was rather opinionated. And he could be very biting, especially with people who didn’t understand his work. I probably would have said, . . . what’s wrong with him?’ ”

Willful, stubborn, unkempt--Turing was that, too. Though brilliant, he stammered like a schoolboy and bit his nails obsessively. Yet when you ask what pleasure McMurtry gets from playing such a character, he notes that Turing’s disagreeable traits presented the sort of challenge that has always drawn him to difficult roles.

This paradox amuses him. “I like doing unattractive people,” says the star of Hugh Whitemore’s biographical drama, which opened Friday on the SCR Mainstage. “I don’t like to play heroes.”

Among his more than 100 roles at San Diego’s Old Globe Theatre, for instance, the slightly built, sandy-haired actor particularly relishes Iago in “Othello,” whom he interpreted as a compulsive liar and sociopath in the guise of a seemingly honest hail-fellow-well-met.

This time, far from portraying a consummate dissembler, McMurtry gives us someone who is constitutionally unable to keep up any pretense whatsoever. Even the exchange of mere social pleasantries is a hurdle for Turing, who feels friendlier toward his computer prototypes--”thinking machines” he called them--than toward people.

There was considerably more to Turing than his defects, of course. A genius at cryptography, he cracked the secret German military code during World War II. Turing’s work, championed by Winston Churchill, enabled the Allies to counter German U-boat attacks, thereby gaining a naval advantage in the North Atlantic without which the war could have been lost.

Ironically, the absence of sham in Turing’s iconoclastic personality--which ordinarily might have been a virtue--proved disastrous. In what turned out to be his worst mistake, he told the police investigating a petty burglary at his home that he was having a homosexual affair. The voluntary admission brought about his arrest for moral turpitude, eventually led to his expulsion from academia and ultimately precipitated his suicide.

“This is the hardest role I’ve tackled,” McMurtry maintains, sipping coffee from a Styrofoam cup during a recent interview at SCR. “I’ve never played this kind of man. He is a genuine eccentric. It’s exciting to be in his body, to see what makes him tick. I almost live the character. I’ve never bitten my nails, and now I find myself doing it all of a sudden.”

In fact, the actor’s nails look manicured, and he seems entirely composed. Casually dressed in a plaid shirt, blue pullover and corduroy pants, he gives not the slightest hint of Turing’s excitability or of his overweening self-absorption.

At 50, McMurtry lives in San Diego with his wife, Terri, and has played leading roles at most of California’s major theaters, including the Mark Taper Forum in Los Angeles, the American Conservatory Theatre in San Francisco and the Los Angeles Theatre Center. He last appeared at SCR in 1986 as the melancholy Jacques in “As You Like It.”

Born in Detroit, McMurtry says he grew up “in the trunk” because both of his parents were show-biz dancers. They toured so often that he eventually went to live with his grandparents in Southern California. Not surprisingly, he shunned the idea of making a theatrical career at first and preferred the idea of becoming a painter.

But he changed his mind around 1960 at Los Angeles City College, where he won a nationwide audition for a scholarship to the Royal Academy of Dramatic Art in London. After training there for two years, “I came back ready to lick the world,” he recalls. “I thought I could do anything. And of course I couldn’t.”

Nevertheless, McMurtry is a great believer in British stage technique. Too many “American actors are lazy,” he contends. They have the impression that “if you think right and feel right, it will come out right,” he says. “Well, it doesn’t work that way.”

To prepare for his role in “Breaking the Code,” McMurtry says he consulted a speech therapist to explore the cause of Turing’s stammer. He also sought the advice of a university professor to understand the mathematical meaning of various terms that figure in Turing’s theories, one of which is given at length in a monologue running to more than three pages.

But for all the surface dryness of Turing’s favorite topics--such as the interface between electronics and higher logic--McMurtry brings the man to life with a strikingly visceral performance. His portrayal of a lonely, abstract thinker resonates with a subterranean force remarkable for its repressed energy.

“I usually try to find what the body does first,” the actor explains, “because the rhythms and speech patterns depend on that. A stammer comes from an emotional place. It has a lot of explosive stuff behind it.

“In Turing’s case his mind just works faster than his mouth. He can’t catch up with himself. So he gets mad and irritated that his words are not coming out. And he is most passionate when he is talking about his ideas. That’s when he gets really emotional.”

As for Turing’s homosexuality, McMurtry purposely avoids the use of effeminate gestures. Although the character candidly asserts his sexual orientation--and several scenes show Turing’s predilection for rough young men--McMurtry gives scant physical indication of the obvious.

“He was not limp-wristed or anything like that,” McMurtry says. “If he were, I’d play that. It’s not a problem. I’ve played raging queens, salad boys, total faggots. But Turing is not an old poof, even though that is how he describes himself.”

Indeed, the most intriguing aspect of the character resides elsewhere. For Turing seems, above all, a tragic figure in the classic sense, driven by pride and overtaken by events beyond his control.

“I see him almost as a Creon image,” McMurtry says, “an exile in a land that doesn’t work the way he works.”

“Breaking the Code” by Hugh Whitemore continues on the Mainstage at South Coast Repertory through Nov. 30. The theater is at 655 Town Center Drive, Costa Mesa. Curtain Tuesday to Saturday at 8 p.m.; Sunday at 7:30 p.m.; matinees Saturday and Sunday at 2:30 p.m. Tickets: $18 to 27. Box office: (714) s957-4033.