George Axelrod once suggested that the big, unanswered question about Neil Simon was whether or not the guy had a flop in him. One might ask the same about Tom Stoppard.

His new play, "Night and Day," has produced lines at the Phoenix Theatre box-office. Another work of his. "Dirty Linen," written a sa romp to celebarte the British naturalization of its director, on Nov. 15 notched up its 10,000th performance at London's Arts Theater. And another Stoppard work, "Every Good Boy Deserves Favour." ran throughout the summer in London and is shortly to be seen on BBC television. Rarely in Britain has a top-quality playwright enjoyed such unstinted commercial success.

But what exactly is Stoppard's secret?One small clue to the answer arose the other day in a radio interview about "Night and Day" with Anthony Howard, a former Washington correspondent for the London press and former editor of the weekly, The New Statesman. In the course of the discussion Howard criticised a parody by Stoppard, inside the play, of an eminent American newspaper.

Within a few hours of the broadcast, Stoppard was on the phone to Hward wanting to know precisely what was wrong with the parody and how he could put it right. Most authors with a hit play on their hands would have shunted aside such criticism. Stoppard, in contrast, sat down to do a 10-line rewrite.

There is sanity in Stoppard's method. He once said that he had never left journalism emotionally, and he has the first-class journalist's ability to get his fccts absolutely right. Before he wrote "Travesties" - based on the discovery that Joyce, Lenin and Tristan Tzara all lived in Zurich during the First World War - he went into hard training by reading and annotating the characters' works. I have a vivid memory of meeting Stoppard one morning on the steps of the London Library (a booklovers' fleshpot) with a huge pile of books precariously balanced between his chin and his left hand. "What have you got there? "I asked. "My new play," he replied. Having just finished "Travesties," he was retiring his homeworK.

Though immensely popular, Stoppard is not a noisily public figure like some of his contemporaries. He rarely gives interviews, shuns fashionable parties and leads a contended domestic life at Buckinghamshire home with his wife, Miriam (a doctor and managing director of a pharmaceutical firm), their two sons and two more sons from his previous marriage. One cannot, for instance, easily imagine Stoppard joining John Oshboarne's Playwrights' Mafia, a club devoted to the physical roughing up of drama critics. Not only has Stoppard little need to do so - since most of his notices are adulatory - but the headlinecatching, gossip-column nature of such an enterprise is the polar opposite of his kind of friendly privacy.

As a dramatist. Stoppard is in a unique position now in Britain: He has bridged the gap between intellectual accalim and popular appeal. He cannot only earn a 5,000-word assessment in a literary monthly like Encounter, but also healthy advance bookings whenever his name goes up on the billboards.

Laurence Olivier once said that "theater is the first glamorizer of thought." Stoppard can take a complex idea, deck it out in fancy dress and send it skipping and gambolling in front of large numbers of people. Shaw did the same, but Stoppard's plays have two things Shaw's rarely possessed: sex and scenic excitement. There was an autobiography published in the 1930s oddly entitled "Ideas Have Legs." I would say Stoppard's achievement is to have shown that ideas not only have legs but a heart, a larynx and genitalia, as well as a sizable popular audience.

"Night and Day," which has just opened to highly enthusiastic reviews at London's Phoenix Theatre, marks a radical change of direction for Stoppard. After the high-flying, word-spinning, light-fantastic "Jumpers" and "Travesties," it offers a straightforward linear narrative. But, more significanly, the dramatist who once said, "I should have the courage of my lack of convictions," has written a play committed to a particular cause: The maintenance, at any cost, of freedom of the press. In the kind of headline-writing Stoppard parodies in the play, it might be billed as Pun-Packed Playwright Pursues Polemic.

The signs of change were detectable in Stoppard's last work, "Every Good Boy Deserves Favour," which brilliantly defied the theatrical law that you cannot simultaneously have your hand on your heart and your tongue in your cheek. Its use of irony, mixed identities, outrageous conceits (not to mention a full-scale symphony orchestra) marked it out as the work of a dazzling high-wire performer. At the same time it was a profoundly moral play about the brainwashing of political dissidents in Soviet mental hospitals.

In "Night and Day," however, the verbal pyrotechnics are very much at the service of the main theme. The play is set in a fictitious African country, Kambawe, where there is a rebellion going on against a dictatorial president. But Stoppard is less interested in African politics than journalistic principles. Two English journalists turn up to cover the crisis, both unfortunately working for the same Sunday newspaper. On the left (very much so) is Dick Wagner, an old Fleet Street pro whose proud vaunt is "I don't file prose. If file facts" and who is deeply committed to worker solidarity. On the right is Jacob Milne, an is the last line of defense for all other freedoms," who hs the last line of defense for all other freelancer who bas fallen foul of his union and who happens to pick up a scoop interview with the rebel colonel.

The play is partly about the differing attitudes to journalism these two men represent, and partly about their involvement with the glamorous wife (played by Diana Rigg) of the mining engineer whose house they are occupying.

Stoppard himself once said, "What's wrong with bad art is that the artist knows exactly what he is doing." "Night and Day" is an exhilarating refutation of that thesis: It's good art (wittiy, stimulating, alive with ideas) but the result of a perfectly conscious intention.

Admittedly I think Stoppard plays less than fair when it comes to the central debate about the press. In Britain there has been fierce argument about the attempt by the journalists' union to establish a closed shop and set up basic qualifications for aspiring scribes. Some see it as an attempt to bring journalism into line with other professions (would you want an unqualified surgeon to take out your appendix?), others as a denial of a fundamental freedom. Stoppard is passionately one of the latter. There is no reason why he shouldn't be (particularly as he is an ex-reporter himself); but, theatrically, the air is filled with the sound of dice being loaded.

But even if the play is stronger on polemic than dialectic, it at least raises vital issues, and does so in an entertaining manner. Stoppard has a matchless ability to weave into a serious debate boffo laughs and knockdown zingers. Thus the dictatorial Kambawe president, boasting that his country has a relatively free press, asks. "Do you know what I mean by a relatively free press? . . . I mean a free press that is edited by one of my relatives."

Of course, not all Stoppard's changes in approach are the result of free choice. Some (such as the scenic simplicity and the reduction of the characters to eight) are dictated by economic necessity. Stoppard's recent extravaganzas have been written for subsidized companies, whereas this is a West End play for a commercial management. As Stoppard remarked to me some time ago. "If I'd written a play demanding 18 jugglers and 27 pigeons in the first two scenes. Michael Godron, the producer, would have thrown it back at me. Even the prospect of using a jeep made him blanch visibly."

But what is important about "Night and Day" is that it demolishes Stoppard's own prejudice against drama that begins with a preconceived idea. After all, "A Doll's House," "Saint Joan" and "Mother Courage" are three examples of masterpieces written out of a sense of burning conviction. And while "Night and Day" may not reach that level, it does at least prove that Stoppard can write about what is closest to his heart without sacrificing his natural dexterity. Passion and wit don't often go together, when they do we should applaud.