On board a ship returning from the US in 1929, Cecil Beaton - then a fast-rising star of British photography - met the actor and playwright Noel Coward. The latter decided to give Beaton some advice on how to present himself. "It's important not to let the public have a loophole to lampoon you," Beaton in his diary recorded being told by Coward. "You should appraise yourself," he went on. "I take ruthless stock of myself in the mirror before going out, for even a polo neck or an unfortunate tie exposes one to danger."

Here is the essence of Coward and his work: constant self-appraisal and awareness of the public response. That Noel Coward was his own finest invention is now widely understood, but he also exercised the same intensive control over every scrap he wrote, so that nothing might be left to chance.

During his lifetime, Coward's extraordinary discipline was concealed behind a facade of apparent effortlessness and the presentation of charm at all times. John Lahr's 1982 study of Coward the Playwright understandably opens with a chapter called "The Politics of Charm", in which Hesketh Pearson is quoted from 1923 remarking "the showman in Coward takes the form of charm, plus excessive volubility. He is `charming' to everybody, and as a consequence, he has hosts of admirers." Charm was used by Noel Coward as an offensive weapon both on stage and off.

His most obvious alter-ego among his plays, Gary Essendine in Present Laughter (1942), remarks at one point: "My worst defect is that I worry too much about what people think of me," which is so winsomely charming that the audience immediately succumbs to this monster of self-regard. At another point in the same play, when Essendine complains that everyone loves him, his secretary Monica retorts: "There's hell to pay if they don't." The politician and diarist Henry `Chips' Channon, a contemporary whose social-climbing was of equal Himalayan proportions, wrote about meeting Coward in 1945 when "he was flattering (he is an arch-flatterer), insinuating, pathetic and nice. I have never liked him so much, though he talked mostly about himself."

Gilda, in Design for Living (1932) at one point asks her devoted but dull admirer, Ernest, if she might be described as a super-egotist, thinking of herself too much and not enough of others. No, he replies, "thinking of other people too much through yourself." The same could be said of Coward as well; although he tried hard to disguise this weakness, as Chips Channon noted, the playwright's preferred subject (in life and in art) was always himself. Inevitably, therefore, just like the characters he favoured in his plays, he had to take charge of every situation.

"Coward was always in control," the actress Anna Massey once said. "You never felt he lost control. And he was always surrounded by loving guard dogs. He never went anywhere alone." The playwright confirmed this impression in a 1966 interview when he pronounced: "You've got to control the audience. They've got to do what I tell them." Loss of control was clearly a terrifying prospect and provides the tension in many Coward plays from Hay Fever (1925) onwards. Despite his best intentions, the effort of attempting to maintain control led him to have several nervous breakdowns, the first in September 1926. His biographer Philip Hoare remarks of this collapse: "Coward had to flee the claustrophobia of his self-created world; if he stayed too long . . . he might discover how thin the ice was beneath his feet."

The ice may not necessarily have been so thin, but it was certainly brittle. Coward's skills as a playwright are best demonstrated when they might appear slightest. He liked to suggest that the creation of each play was as undemanding as its content, whereas considerable hard work and substance always lay beneath the laconic surface. A tough disciplinarian, he rose early every morning - no matter where he was living - and worked steadily until lunch. Publicly, Coward liked to say he had tossed off a play in just a few days - three for Hay Fever, four for Private Lives - but this was to overlook the vast amount of time spent in creative preparation.

In the same way, plays such as Design for Living or Private Lives, despite being relatively plot-less, are actually tightly plotted. Little happens but what does, happens in a well-ordered way. Similarly, Coward dialogue is intensely drilled and stripped back. It is another indication of his disciplinarian nature that he would always pare back scripts, allowing John Lahr to describe Private Lives as being "minimal as an art deco curve."

In an exchange typical of the play, its two leads, Elyot and Amanda, are discussing his trip around the world.

"Amanda: China must be very interesting.

Elyot: Very big, China.

Amanda: And Japan

Elyot: Very small."

Other than good timing, no more is needed here to ensure a moment of impeccable comedy. In the same fashion, elsewhere in the same play, one of the greatest audience laughs has traditionally come from the line: "Very flat, Norfolk."

Much of Coward's success depends not just on delivery but also on context and subtext. Elyot and Amanda's conversation in Act One of Private Lives is deliberately dependent upon surface banality because what matters - their mutual love - must remain unexpressed. The same is true of one of Coward's few enduring serious works, the 1945 film Brief Encounter in which Laura and Alec also convey love for one another without any overt statement of their feelings. At one stage, Laura comments: "Do you know, I believe we should all behave quite differently if we lived in a warm, sunny climate all the time. We shouldn't be so withdrawn and shy and difficult."

The notion of Coward being either shy or withdrawn is clearly ludicrous, but he was always intensely careful about what personal information might or might not be shared with the public. Hence the development of his familiar sophisticated persona: this provided a glittering, impenetrable carapace beneath which he could take shelter. As Leo - the character he played in his Design for Living - says, "It's all a question of masks, really; brittle, painted masks. We all wear them as a form of protection; modern life forces us to. We must have some means of shielding our timid, shrinking souls from the glare of civilisation."

At the age of 70, Coward admitted in an interview: "I have taken a lot of trouble with my public face." In his admiration of hard work, his ambition and his belief in meritocracy, he was distinctly un-English but chose to keep these qualities concealed beneath the same veneer of insouciance displayed by many of his characters.

Writing in 1992, Jenny Uglow remarked "To enjoy Coward is to relish artifice and polished surface, as he dictated; the depths below hold disenchantment." Much earlier, reviewing Noel Coward's first volume of autobiography, Present Indicative, published in 1937, Cyril Connolly suggested the playwright on this occasion engaged in uncharacteristically dull dissembling "because he has so much to conceal behind the smoke screen."

Perhaps this is to ascribe too much psychological baggage to a man who freely admitted his intellectual limitations (and all his life loathed intellectuals such as Connolly) and avoided using plays to convey ideas, saying "I don't like propaganda unless it's disguised so brilliantly that it is entertainment." Even in his diaries, Coward tended to avoid too much self-analysis, preferring to use them as another opportunity to display his public persona.

As Uglow notes, with Noel Coward surface is best; it is what most preoccupied him and where the most distinct impression of the playwright may be found.

A new production of Coward's Private Lives opens in London at the Lyttelton Theatre (Royal National Theatre) on May 13th. The Festival of Music in Great Irish Houses will hold an evening dedicated to Coward at Carton House on June 18th.