In recent years theatre has sought assistance from a pair of popular art forms. Shows based either on movies, or on pop groups’ back catalogues, have become mainstays of the theatrical economy. So the latest musical to open in the West End has the whiff of  boardroom cynicism. What happens when you randomly select a famous film and an iconic songbook, yoke them together and shove them out in front of the footlights?

People sneer at musicals for endless reasons: they hate Broadway brashness, non-naturalistic lurches in and out of song, the sentimentality. One of the least acknowledged reasons, however, is because their plots – predictability plus songs – have zero tension. And you know what? Placed in the witness box, many a musical emerges guilty as accused. But the quietly astonishing Once is innocent of all those charges.

It’s one of the most anticipated theatrical openings of the year, with tickets allegedly changing hands for astronomical sums and some pundits rushing to issue dire warnings of the depths of its lewdness and its shattering shock factor well before its official first night. So can this musical by Robert Lopez and the incorrigible South Park creators Matt Stone and Trey Parker possibly live up the hype? The answer – rather like the existence (or not) of some supreme guiding deity – depends on your point of view. 

This stylish, witty musical celebrates the 50-year love affair between the first openly gay film star, William Haines, and Jimmy Shields, a set decorator. It embraces the fashion of the Twenties, the design of the Thirties, the glamour of the big film studios, and the freedom of unconventional lifestyles. A compelling story, fine tunes and some rather attractive actors make for a highly enjoyable evening.

Even singular sensations grow older - yet A Chorus Line, which coined the phrase, seems ageless, so sure is it of its place in musical theatre history, so locked now into our theatrical consciousness.

A Chorus Line is one of the great American musicals. It opened off Broadway in 1975, rapidly barged a path to a larger Broadway house and proceeded to run for over 6,000 performances, breaking records along the way. Chicago, which opened in the same season, failed to seize the city's imagination in the same way, and had to wait till the 1990s to find an audience prepared to devour it. At the Tony Awards the musical about the foot soldiers of showbiz, the faceless dancers high-kicking in line, went on to win nine gongs, and then picked up a Pulitzer Prize.

It's odd that Jerry Herman merits only a passing mention in Stephen Sondheim's two-volume autobiographical take on Broadway words and music, Finishing the Hat and Look, I Made a Hat. In a couple of subjects Herman chose no less daringly than the master. Yet while La Cage aux Folles is now so entrenched that we forget its original boldness in asserting a loving gay relationship, Dear World's eccentric mix of eco-plea and nostalgia has yet to be established as a bittersweet chamber piece.

Les Misérables is revolutionary, but not in a French way. Oscar-winning director Tom (The King's Speech) Hooper’s film of a musical seen by over 60 million people in over 40 countries and in half again as many languages has engaged so much critical ink I’m almost dreading writing my own opinion. However, as a property that has run onstage for 27 years, Les Misérables - once nicknamed The Glums - is a stirring tale of love, loss, cruelty, salvation and predation that also comes with a built-in audience of which you may or may not be a member.

Cinemagoers with an aversion to musicals need not fear, as in Pitch Perfect most of the singing is in a sane context, rather than its characters breaking into lavish routines in the street. After the fun but exhaustingly naff Rock of Ages, this comes as something of a relief. And if its chart pop mash-ups and campus antics seem squarely targeted at the teenage and twenty-something market, Pitch Perfect broadens its appeal shrewdly with some cross-generational acerbic and offbeat humour.

In 1972 John Kander and Fred Ebb were invited by Bob Fosse to a private screening of his film version of their hit stage musical, Cabaret. The movie starred their protégée, Liza Minnelli, who at only 19 had won her first Tony in Kander and Ebb’s first show, Flora the Red Menace, and for whom they would go on to write “New York, New York”. “Liza was our girl, and we cared very deeply about her. We sat there afterwards and didn’t know what to say to these people whom we liked so much. Because we just hated it.”