A funny thing happened...

The musical that will be revived at the National on 9 July is an extremely silly show about stereotypical characters. It has no social or political significance whatsoever, and not a moment of Weltschmerz or a scrap of angst. The songs are by Stephen Sondheim. And, no, that last sentence is not a mistake.

The musical that will be revived at the National on 9 July is an extremely silly show about stereotypical characters. It has no social or political significance whatsoever, and not a moment of Weltschmerz or a scrap of angst. The songs are by Stephen Sondheim. And, no, that last sentence is not a mistake.

A Funny Thing Happened on the Way to the Forum (1962) is an oddity in the Sondheim catalogue for other reasons as well. The first show for which Sondheim wrote both music and lyrics (he had already collaborated with Leonard Bernstein on West Side Story and Jule Styne on Gypsy), it won Tony awards for best actor (Zero Mostel), best supporting actor (Jack Gilford), best librettists (Larry Gelbart and Burt Shevelove), best director (George Abbott) and best musical. But Sondheim was not even nominated, his songs having been deemed pleasant but irrelevant.

Furious at the time, he later credited Shevelove and Gelbart with the success of the show, saying that he had, after all, granted the former's request to write, as Meryle Secrest put it in her biography, "songs that could be removed without making the slightest difference". Edward Hall, however, directing the revival, which will star Desmond Barrit, says, "Oh, no they can't - he was being modest."

Whether you agree with Hall, or think, "Oh, yes they can", there's no doubt about the appeal of this pantomime for adults. The original production ran for nearly 1,000 performances (far longer than any subsequent Sondheim show) and the show was revived twice on Broadway: in 1972, with Phil Silvers and in 1996, with Nathan Lane. (Silvers, who had been the first choice for Pseudolus, the Roman slave who constantly outwits his masters, had turned it down, saying he had spent enough time playing Sergeant Bilko.)

A London version with Frankie Howerd (who reminded one critic of a cynical horse and another of a calculating camel) was a hit in 1963 and again in 1986. Barrit, who saw both, thinks for a moment when asked how Howerd had played it differently the second time, then replies: "Slower." Barrit, however, is not being given the option of ambling through the role. "It's the most physically exhausting thing I've done in my life," he says, "like running a marathon - in circles."

When the show is over, Barrit certainly feels as though he has run seven times round the seven hills of Rome, as one poor soul, duped by Pseudolus, is made to do. He has to pelt through doors and up and down stairs, his feet moving almost as quickly as his geometrically enlarging deceptions that drive the plot.

The story comes from three plays by Plautus, the Ray Cooney of Rome two centuries before Christ, who was the first to write about erring mortals rather than gods - other Plautine comedies inspired The Comedy of Errors and Molière's The Miser. Besides the manipulative Pseudolus, the characters include the ineptly lecherous Senex, married to Domina, whose name also sheds some light on her personality. Their son, Hero, loves Philia, who is promised to the boastful soldier Miles Gloriosus (also the name of another comedy by Plautus). When Miles swaggers into town, he sings, in direct translation from Plautus: "I am a parade!"

"Larry Gelbart says, every time he hears the audience laugh at that, he can't get over it," says Hall, "people laughing at a 2,000-year-old joke." The fresher ones are good, too. Pseudolus tells off an impertinent eunuch with: "Don't you lower your voice to me!", and Senex urges others to take from his marriage an important lesson: "Never fall in love during a total eclipse." Apart from its title, the play never indulges in anachronism, but a sharp, late-20th-century New York sensibility underlies its humour. While not overtly cynical, it exempts no one from ridicule. Instead of being the classic pair of sweet-but-boring lovers, Hero and Philia are a pair of birdbrains - though Philia is a bit more backward than her boyfriend. Assuring him that her heart will remain his, though everything else belongs to Miles, she sings, "When I kiss him, I'll be kissing you, so I'll kiss him morning and night - that'll show him!"

Though praised for the modernity and maturity he brought to musicals, Sondheim turns the clock back in this show, it is generally agreed, to a time before songs were integrated into the story and advanced character and plot. (Hall, again, thinks otherwise: "The songs put fantastic detail on the ideas so you can visualise them, and, in doing so, they progress the story.")

But, during the out-of-town try-outs, one of his numbers did something even more important than becoming a popular hit: it saved the show from closing on the road. Despite its hilarious script, Forum was dying, one night even playing to an audience of 50, when the director Jerome Robbins was brought in to doctor it. He diagnosed a need for an opening song that would make it clear, from the first, what kind of show the spectators were about to see. Sondheim came up with "Comedy Tonight", a shamelessly hucksterish number that sold the show to its audiences so well, they immediately changed from apathetic to ecstatic. ("They don't even", Barrit says, "have time to catch their breath.") This witty version of a variety-show opener trips along until the end of the bridge, when it slouches into a Mae West-style come-on, revelling in its own shamelessness. Miles' triumphal march steals from up-tempo good-time songs as well as pompous Hollywood music when he announces: "There are lands to conquer, cities to loot and people to degrade!" The one show-stopper, "Everybody Ought to Have a Maid", highlights the jokes with burlesque beats: "Oh! Oh! Wouldn't she be delightful, sweeping out (boom!), sleeping in ( boom!)."

In fact, the whole show can be seen as a Valentine to the trouser-dropping school of comedy; a style that was vastly popular in silent films, vaudeville and burlesque (which, of course, was the subject of Sondheim's previous show, Gypsy) but was breathing its last when the creators of Forum were kids. Unlike those of our more sensitive era, comedians then were often thought funny in proportion to their poundage, and the notion of a fat slave (as Zero Mostel was) was inherently comic. Barrit now looks, let's say, a good eater, but he used to be quite fat. "No, you needn't qualify it. I was fat. You could have got all the girl dancers in this show into me and still have had room for a few more." Fatter is funnier, he agrees. "When you're fat, you can get away with more. People are more forgiving, and they find you more lovable. Even when fat people say nasty things, like Robert Morley playing Sheridan Whiteside, people think, 'Isn't he cuddly?'" There are comics, he acknowledges, who use their bulk in an aggressive way, but in this show Pseudolus never has the chance. "I get manhandled quite a lot," Barrit says, so he is able to do the big-but-helpless sort of comedy as well.

Hall thinks the mainspring of the comedy is the characters' desperation: "Everybody has something to hide, and something to lose. The more they have to lose, the funnier it gets." (Top of the list is Hysterium, perpetually in a state through being blackmailed by Pseudolus, who knows about his secret collection of erotic pottery.) Barrit also worries about what he and the rest of the cast could lose through their nightly workouts. "Now, you will come and see us early in the run, won't you?" he asks. "Do, because if you wait too long, you might not see us at all."

'A Funny Thing Happened on the Way to the Forum' previews at the Olivier, National Theatre, London SE1 (020-7452 3000; www.nationaltheatre.org.uk) from 28 June and opens on 10 July, as part of the £10 Travelex season