It has been said that the most revealing part of a play is often not the rehearsed action on stage but what comes afterwards — the curtain call, when characters evaporate allowing a tantalising glimpse of the actors who play them.

But imagine how much more would be revealed if we knew what passed between the creators of a show. The love, the loathing, the gratitude, the resentment and even, sometimes, the betrayal. Much of this is communicated in the form of letters and, more recently, emails, many of which in the case of the National Theatre, are stored in archives and are now the subject of a new book.

The idea for Dramatic Exchanges came to theatre historian Daniel Rosenthal while researching his huge biography, The National Theatre Story.

“I can actually pin-point it to a specific moment,” remembers Rosenthal. “I had been shown in to the National’s archive and there was this very fierce exchange of letters between Peter Hall and Peter Shaffer about [Shaffer’s ] Amadeus.”

Hall had directed the play to great acclaim in London and New York, earning “hundreds of thousands of dollars” in royalties. He desperately wanted to direct the film version of the play and had dearly hoped Shaffer would insist to the film’s American producers that he get the job.

However, then Hall discovered in the press that the film had gone to Milos Forman. Shaffer wrote to Hall in July 1981 to apologise. “I dreaded too much to be the one to tell you,” he wrote. “In shrinking from causing you a moment of inescapable distress, I had actually caused you much greater distress,” before begging Hall to forgive him.

Hall’s reply was as forgiving as granite. Dated nine days after Shaffer’s, Hall’s letter begins “Please excuse the silence. I needed time,” and then pulls no punches. He calls Shaffer a coward and then says “I do not accuse you of lying. It’s worse in a way: I believe you deceive yourself.” Then this titan of British theatre, who founded the RSC before running the National Theatre, and who introduced Beckett to the London stage, declares himself to be “very, very hurt.”

It turns out that even the highest achievers are as vulnerable to human frailties as the rest of us.

“It’s not voyeuristic,” insists Rosenthal about his book. “It’s about understanding the extraordinary passion that writers, directors — all artists — bring to their work.”

Tony Kushner, whose Angels in America is now held to be one of — if not the — finest piece of theatre writing of the past 50 years, is no exception. Set in Reagan’s America, Angels explores the state of a nation primarily through characters who are gay, Jewish and African American during a time when the country was plagued by HIV. Yet Kushner was full of self-doubt, it turns out, especially In 1993 while working on Part Two of the work, Perestroika.

“If you were to interview Tony Kushner today,” says Rosenthal, “and say, I know when you were trying to finish Perestroika it was difficult, can you describe that for me, he would be able to recall it but not in the same way as being in the moment. We are all the same. Most of the time to remember is to paraphrase.” But the immediacy of that time jumps right of Kushner’s letters.

“I am still struggling with the play,” Kushner writes to director Declan Donnellan and designer Nick Ormerod. “The changes in my life since the play [Part One] opened [to great acclaim] last January in London have caught up with me; I’m scared about the future and of the reception of my work.” He needn’t have been, of course.

The following year, none other than Arthur Miller would write to the British director David Thacker about the forthcoming UK premiere at the National of Miller’s most explicitly Jewish work Broken Glass. The play is about the paralysing effect that reports of Kristallnacht have on a Jewish woman in Brooklyn in 1938. The play had already opened in America but, although it was an inferior production to the one that Thacker would direct in London, it was still up for a Tony award.

“The play has built a wonderful reputation…” writes Miller to Thacker in the Spring of 1994. “I myself doubt we’ll beat out Perestroika for the Tony but nobody yet knows for sure.” Four days later, Perestroika won Best Play which must have put some of Kushner’s anxiety to rest. Just as fascinatingly, however, are Miller’s notes to Thacker in the run-up to the London opening of Broken Glass.

“The title, for Jewish people, will also signify the smashing of a glass giblet under the heel of a groom, indicating the sealing of the marriage,” explains Miller. “But it also signifies the destruction of the Temple by the Romans so that Jews will never forget it. ‘If I forget thee, O Jerusalem may my right hand wither...’

“Of course,” continues Miller with tongue in cheek, “the latter connection will probably not be made by non-Jews, but to hell with them.”

Only a couple of months after Miller’s play opened on the National’s Lyttelton stage, a new dramatic force announced itself at the Cottesloe in the form of Patrick Marber’s Dealer’s Choice. The National’s then director Richard Eyre had discovered Kushner and now he had done the same for Marber. Two years later, Marber followed up that success with what is now a modern classic: Closer. As it began rehearsals in New York, Marber wrote to acknowledge Eyre’s role.

“Dear Richard, Rehearsals start on Monday for NY Closer. I just wanted to tell you how much your faith in me/my work has changed my life. None of this would be happening if it weren’t for you. I’m more grateful than I have the talent to express.”

However, the same success did not meet Marber’s most Jewish play Howard Katz (2001), about a suicidal Jewish showbiz agent (played by Ron Cook). It wasn’t well received by many of the critics. “I must admit the critical response has been surprising,” writes Marber in a letter to the National’s new artistic director Trevor Nunn, “because, to me, the play is clearly my best. Then again, what do I know?”

And that’s one of the lessons of this book: no matter how big a past success, uncertainty always waits in the wings for the next play. “My progress is slow,” says Tom Stoppard in a letter to Eyre about his A E Housman play The Invention of Love. Eyre had been waiting for a while for the script and it would be another year before it would eventually reach the stage in 1997. But the masterful Stoppard was having problems with the structure. “…I’m resisting some kind of interlocutor between the story/events and the audience… In a nutshell I don’t know if I’m writing Arcadia or Travesties and until I do know, it won’t take off.”

“I get great pleasure from what I call the practitioner as theatre-goer letters,” says Rosenthal. “What does Ian McEwan think of [Michael Frayn’s] Democracy?; What does John Le Carré make of Ian McKellen as Richard III? These very gifted and distinguished artists in their own right commenting on another form. I find that very interesting.”

And then there are the letters from one theatre person to another; from Alan Bennett apologetically writing to Nicholas Hytner about the amount of work there still has to be done to his latest script for The Habit of Art. “Was it for this, you must be wondering that you were delivered out of Zion.” Or the email sent to American playwright J T Rogers by his friend, the playwright Ryan Craig, to whom he’d send a script of Oslo, surely the best play about the Israeli/Palestinian conflict ever written.

“My God, it’s a good play,” begins Craig, author of such Jewish-themed plays as What We Did to Weinstein, The Holy Rosenbergs and Filthy Business. “I mean it’s a really good play. I mean I loved it from the very first word to the very last. I was reading it over a cup of English Breakfast Tea in a crowded cafe and only too late realised I was occasionally slamming the table shouting ‘Yes! Yes!!’ Like some insane, bookish, bald Meg Ryan.”

‘Dramatic Exchanges: The Lives and Letters of the National Theatre’ is published by Profile Books