There are several moments in Hugh Whitemore's "Breaking the Code" when Derek Jacobi fulfills our conventional expectations of a major actor. He expresses pain, anger, grief, desolation and, once or twice, a curious mix of them all, making one's eyes pop at the authenticity of his emotions and the potency of his performance. And yet it's his playing of the small moments, not the big ones, that we should perhaps be treasuring the most. Again and again he reminds us that the art of acting is the art of conjuring much out of little, or more out of less, or something out of nothing. It's the art of making silence articulate and platitude eloquent. It's the art of taking a line as flat as a parking lot in a small Midwestern town and somehow suggesting that there's an abundance of animal life in the fissures and crannies beneath it's surface.

The contemporary actor most obviously adept at maximizing the minimal is of course Laurence Olivier, who was Mr. Jacobi's employer and mentor when he was an apprentice actor at Britain's National Theater in the 1960's. No one who saw Sir Laurence's Macbeth in 1955 is likely to forget the power he found in the three tiny words with which he greeted Banquo's murderers-to-be. "Well," he said, summoning them to his side with a smile and a crook of the finger. "Then," he added, firmly pointing out where he wanted them to stand. "Now," he finished with a sudden, curt and unanswerable authority. "Well then now"--and Macbeth's lackeys were ready to commit any atrocity he might demand.

That's admittedly an extreme example, displaying a wider gap between word and meaning than you'll find in "Breaking the Code." Mr. Jacobi is an audacious actor, but he can't match Sir Laurence (who can?) when it comes to creating outrageously unexpected effects onstage. Again, he is not playing Macbeth, but Alan Turing, not a crazed medieval tyrant bloodily at odds with God and humankind, but an eccentric modern mathematician and computer pioneer in trouble with the authorities because of a homosexual indiscretion. Yet if his invention is less sensational, it's also more consistent. He spends most of "Breaking the Code" onstage, and much of his time onstage speaking. And he seems incapable of uttering a sentence without filling it with all the thought, all the feeling, all the subtlety, all the LIFE which it can possibly contain.

Consider, as a sort of case study, Act One, Scene Five: It's wartime Britain, a country house north of London. Here, scientists are struggling to unscramble the Nazi codes, and most particularly those being dispatched via the Enigma machine to the U-boats imperiling the merchant convoys in the Atlantic. If the war is to be won, it's imperative that this device be penetrated and broken. Enter the precocious Turing for a job interview with the institution's boss, a genial old mandarin called Dillwyn Knox, played by Michael Gough. "So you found us all right," says Knox. "Yes, thank you, no problem," replies Turing. And for a time that's very much the conversational level: unsurprising, bland, even banal, and dramatically effective only insofar as it feeds us the background information we need to absorb if we're to understand what ensues.

Yet that is the impression one receives from the page, not the stage. With Mr. Jacobi on one side of a table, Mr. Gough drooped over it or perched on it, and electricity flowing quietly between them, Mr. Whitemore's understated script becomes not just real, but fascinatingly real. One might at first wonder if this is because Turing's mannerisms are being presented in such rich and arresting detail. Mr. Jacobi gasps and stammers, snorts and whoops, gives little gauche titters, and at one point honks weirdly through his nose like a migrating goose. As if that weren't enough, he wrinkles his nose and scratches at it, nibbles at his nails, bends down to fiddle with his shoelace or rub his knee. But that can hardly explain the scene's growing tension, because we've already observed plenty of awkward, clumsy social behavior from the same source. We know that Turing, the mathematical genius, is also a post-adolescent muddle and mess, with his stuttering consonants and nervous tics, his baggy pants and ineffably shabby tweed jacket.

But that's only the external Turing and, in this instance, pretty obviously the outer expression of an inner unease. if one watches Mr. Jacobi carefully, one can see that he's carefully studying Mr. Gough's Knox. After all, Knox is in charge of a lab where Turing knows that both his intellect and his imagination are going to face challenges greater than any they've confronted before. So he is anxious to impress yet resentful at having to impress, cooperative yet deeply doubtful about the mental caliber of the man on the other side of the table. "There's no need to be alarmed," says Knox as he opens Turing's file. "I'm not," replies Turing a little too abruptly, his head suddenly moving forward and then backward, like a tortoise wondering how far it's safe to emerge from it's protective housing.

But this isn't a one-man scene; far from it. Mr. Gough is acting too, and acting very well. He looks like a benign old mole, blinking half-comprehendingly at the unaccustomed light. There's something vague and even lost about his manner, and something oddly off-the-point about several of his remarks; and yet one senses that he's actually listening most earnestly. Is he exaggerating his inadequacy? Is he trying to reassure and draw out Turing, who eventually admits he's worried about coping with institutional discipline? If so, the strategy is successful, because the mathematician's confidence visibly increases as the scene proceeds. When Knox asks him about his interest in codes, Turing leans forward and momentarily becomes dreamy and nostalgic, remembering and sharing a boyhood enthusiasm. When he's asked how he reconciles his new job with his one-time pacifism, he allows his "well, I'm here" to sound just a little mocking and patronizing. A minute or two later, he's exasperated and even angry, one hand jabbing forward as he emphasizes his patriotism and his eagerness to battle the Enigma, the other betraying a continuing insecurity by absently scratching at his leg.

Yet by now Mr. Jacobi's Turing is almost relaxed enough to launch into one of the play's longest and most important monologues. Knox hasn't exactly won his trust. Indeed, one senses that Turing feels a slight contempt for this shambling, apologetic and sadly unscientific old man. But Knox has achieved something more important, which is to allow Turing to feel his superiority and even assert a certain toughness of spirit. So, grudgingly and suspiciously at first, but with accelerating pace and excitement, Turing talks of the art of math. A splutter of weary derision at the public's ignorance of the great theoreticians, a triumphant shout of laughter as he describes the complexity of his own work, then a moment of gravity and quiet as he comes to his most significant contribution to date: the conception of a thinking machine. Who would have thought so abstract a subject could excite passion so intense, mobile and various?

Mr. Jacobi is an actor who conscientiously refuses to preplan a performance, trying always to react as spontaneously as he can to dialogue and events that are, after all, supposed to be occurring for the first time ever. Consequently, he may play the same moment in the scene quite differently from night to night. When I saw him in London, he spoke the line "Godel's theory is the most beautiful thing I know" with a kind of rapt awe, like a worshipper at a shrine; at the Neil Simon Theater recently it was more a half-embarrassed mumble, and he played with his shoe as he spoke, as if the subject were too sacred to share with an unbeliever like Knox. That, too, is a sign of a major actor: a willingness to experiment, up there, on the stage, in front of the audience; a recognition that reality isn't fixed and unchanging, but can shift with whatever emotional winds have been generated during the evening.

And there's plenty more that Mr. Jacobi manages to find in that downbeat encounter in Act One, Scene Five. For instance, his Turing tells Knox he's spent the morning before the interview seeing "Snow White" in the local cinema, and in two sentences manages to mention both that its heroine eats a poisoned apple and, flashing a tiny smile as he speaks, that its ending is "quite touching." Just for a moment one notices an innocence, sweetness and vulnerability inside the uncouth intellectual. It's incongruous, it's interesting in itself, and it's also important, because at the end of "Breaking the Code" Turing will kill himself by devouring an apple laced with cyanide. It is an act that may be despairing, but may be an escape into what he perceives as a fairy-tale world of imaginative thought beyond the restrictions of the adult body. Perhaps Mr. Jacobi is preparing for the future as well as inhabiting the moment.

Certainly, he's enriching the character even further. Imagine an amalgam of Einstein, Peter Pan, the Emperor Claudius, a scarecrow and a bum---and, no, you've scarcely begun to describe Turing as he surreptitiously emerges in what, believe it or not, is still just a short interview for a job.