"It's marvelous," says Alec McCowen about the schizophrenic show -- half poignant realism and half knockabout comedy -- that has brought him and a company of actors from Britian's National Theatre to Baltimore. "The only sad thing is I don't think the public sees the best part -- which is intermision, when I change from a dessicated old schoolmaster into my scarlet tights as Romeo."

He enuniciates his words with knife-edged precision, proving that he has come a long way since the days when a broad Kentish accent made him -- in his father's opinion -- a doubful prospect for the stage. At 55 or thereabouts, McCowen is one of the most respected successors to the generation of Laurence Olivier, John Gielgud and Ralph Richardson. And by doing two such incongruous roles in a single evening, he sustains a quick-change tradition in English acting that reached its height in 1946, when Olivier paired Sophocles' "Oedipus" with Mr. Puff in Sheridan's "The Critic."

But at 40, he was still a supporting player, and his career was on a downward curve. He had played Touchstone in "As You Like It" and Barnaby in "The Matchmaker." He had spent 18 "very depressing" months with the Royal Shakespeare Company playing the Foot to Paul Scofield's "Lear" and Antipholus in "A Comedy of Errors."

His days as a juvenile were limited, he told himself, and he had yet to convince producers or audiences that he could play a full-fledged grown-up. "Already I was having to brown-in a bald spot and shade away a sagging chin," he recounted in "Double Bill," a book of theatrical memoirs published last year. In 1966, the RSC offered him another fool's part -- Feste in "Twelfth Night." He turned it down, and struggled through a period of unemployment, TV work and stage flops, while he scoured the horizon for a chance to break into theatrical middle age.

And along came "Hadrian VII," Peter Luke's adaptation of the cult novel by Frederick Rolfe (alias Baron Corvo), about a half-crazed aspiring cleric who, by a miracle, becomes the first English pope of modern times. "I never had a part that was quite such a virtuoso part as Hadrian," says McCowen. "I think if I couldn't succeed in that, I really wouldn't have a right to complain ever again."

"Hadrian" came to London in the spring of 1968. It and McCowen were instant sensations, and the phenomenon repeated itself on Broadway that fall.

"Hadrian" was an epic production, with its strange personal story set against the color and ritual of the Vatican. Ten years later came the second great triumph of McCowen's stage career, "St. Mark's Gospel," one of the sparest and simplest pieces of theater ever attempted. "I could do it in here," says McCowen -- "here" being the crowded fish establishment on New York's Madison Avenue where he was interviewed over lunch several weeks ago.

"Get me a table and three chairs and I wouldn't even need a stage manager," he says proudly. "St. Mark's Gospel" is pure storytelling; it's "what happened next? And then? And then? That is drama, isn't it, and all the pretty lights and everything are extra."

Speaking "St. Mark's Gospel" -- in modern dress, and from memory -- was McCowen's idea. He had the idea in 1976 and worked on it, irregularly, for 16 months before he began giving public performances in the winter of '77-'78. He encountered a good deal of skepticism. After his first performance in Newcastle, for example, a man burst into McCowen's dressing room and said, "I'm a painter. I want to ask you one question."

"What is it?" McCowen replied.

"Why the hell did you learn it?" said the painter. "Why didn't you just read it?"

"Why don't you just take photographs?" said McCowen.

"Point taken!" said the painter. "I'll buy you a pint."

Three years later, requests for "St. Mark's Gospel" have so outstripped McCowen's ability to fill them that he and producers Arthur Cantor and Gree Garson ahve anointed two actors to follow in his footsteps. Both Michael Tolaydo of the Folger Theatre, and Eric Booth, a descendant of John Wilkes Both, auditioned for the opportunity. Tolaydo "was a mile ahead of everyone else because he'd gone and learned it through," says McCowen. Booth, who will be appearing at the Terrace Theater June 16-28, won McCowen over when he was asked if he really wanted to do "St. Mark." "Passionately!" he answered.

McCowen's sporadic film career includes two noteworthy roles -- the detective in Alfred Hitchcock's "Frenzy" and a liberated fuddy-duddy in the movie of Graham Greene's "Travels With My Aunt." But he is not the sort of star strangers besiege on street corners. "I don't think I have a very memorable face," he says. "I think I share that with one or two other very good actors, like Alec Guinness and Tim Curry." He pauses a moment to consider the pros and cons, and renders a verdict:

"I feel it's a great pleasure to have a face like that."