It's always been said that in the West End, as opposed to on Broadway, theatre reviews don't really matter that much. There are just too many newspapers and too many critics for a single one to bring down a curtain. Possibly, this is true. How else do you explain the roaring success of, say, various Lloyd Webber musicals that have earned nothing but lofty sneers from the critics? Or the dud notices for the hit show We Will Rock You? Conversely, witness other shows that achieved brilliant reviews but couldn't pull in the punters. At which point in the argument the example of the doomed yet universally acclaimed musical City of Angels, starring Roger Allam and Henry Goodman, is usually trotted out.

However, now that theatre tickets average around £40 for a decent seat, and competition for evening entertainment has never been fiercer, this situation is changing somewhat. Needing to be reassured that what they are going to see is going to be good, people have abandoned the old "suck it and see" experimental verve with which London theatre used to be associated. Word of mouth is all very well, but takes too long. With West End rents, shows need to take off immediately, and the only thing that can power the box office is the reviews. That one of the hottest tickets in the West End at the moment is Derek Jacobi in Don Carlos, a supposedly uncommercial German classic about a Spanish prince, is testimony to the scintillating reviews it earned.

My friend Emer Gillespie is currently riding high on the effect of critical acclaim. She is one of five actors in Tejas Verdes, a drama about the Pinochet years, at the off-West End Gate Theatre. I think she was expecting the show to be a modest hit.

The day after the show opened and the first notices trickled out, the show's director, Thea Sharrock, burst into the communal dressing room. "What have our show and the Guardian got in common?" she yelled to the five astonished women who were just beginning to get changed. "They both have five stars!" Indeed, the paper's esteemed critic Michael Billington had delivered a five-star review.

"That afternoon, we sold out for the entire run," says Gillespie. The show has since been extended twice and is now running until Easter. The Gate estimates it has already turned away 600 punters. "Out of eight reviews, we had seven glorious ones," continues Gillespie. "The only bad one was from Time Out. And the day Time Out was published, a very snotty woman rang up the box office and said she wanted to send her tickets back because she had just read it."

Another small but significant change in the system is the use of stars, the very thing that so delighted Sharrock. I think this began on the arts pages during the Edinburgh Festival, where it is vital signposting for people ploughing their way through the Fringe. Its use in the West End, however, is hotly contested. It's great for the company if the reviewer gives five stars. And it's great for the reader if the reviewer gives no stars, because you know a good whipping is bound to follow. But does the reader bother with all the credit-card clicking for a show that is given, say, four stars, let alone two?

Two stars is worse than no stars. It says: "This is a rubbish show, but they tried." And frankly the box office is never going to take off with that sort of character reference.