The film actress Emily Lloyd pulls out of the new Pygmalion during rehearsals. There's panic at the producer's office. Every available young actress is up for the part. A 24-year-old drama student learns the entire role of Eliza because she doesn't know which scene she will have to do at the audition. They like her. But can a drama student walk into a lead in the West End? Or will the critics - rather like the ludicrous Hungarian phonetician in George Bernard Shaw's comedy - expose her as an imposter?

From her first cockney "Nah, then ... " to her final grandly dismissive exit line to Professor Higgins, Carli Norris's performance turns into a two-hour ad for her drama school (Rada). If anything, having an unknown actress play Eliza Doolittle adds just the right subtext. As Norris makes the transition from the sullen scowls and whines of the flower girl to the imperious grace of the later Eliza we are more nervous than she is. Back at Wimpole Street, after the society dance, she peels off her gloves and listens to Higgins's shameless self-approval. Her impassive oval face suggests - without spelling anything out - that she is mad. Someone - other than the professor, that is - has been teaching her well.

These days, the test of a good Pygmalion is whether or not you can sit through the speeches without wishing the cast were going to break into song. It's a question of energy. The director of Pygmalion, the third director on this troubled show, is the veteran farceur Ray Cooney. He presents Shaw's comedy, which rivals Wilde in its paradoxical satire of society's po-faced values, as if it is a brisk and ebullient comedy. Anything else is a bonus. Christopher Woods's designs move us quickly from place to place: the classical pillars of St Paul's, Covent Garden become the pillars in Higgins's library. In the National's showy revival, at the Olivier in 1992, we sat and watched the taxi take Eliza home. It looked great, but emptied the energy out of a proscenium play. Here, the cast just get on with it, in the bright, buoyant manner you might expect. Shaw, after all, is the star.

As Higgins, Roy Marsden accentuates the professor's selfish, gawkily adolescent side. He has tousled hair and rubs his shoe on the back of his trouser leg. Here is a man who has found a new toy: her name is Eliza Doolittle. He is expertly supported by Moray Watson as Colonel Pickering. Watson is the sort of vanishing West End actor - light and precise - who makes sure you hear all five syllables in "particularly". Michael Elphick is excellent as the beaming, portly Doolittle, mining a very funny vein of outrage when he is elevated to the middle class. Barbara Murray glows with tolerance as Mrs Higgins, while, as the disapproving housekeeper Mrs Pearce, the hand-wringing Marcia Warren shuts her mouth with the same finality with which she shuts the library doors. The successful casting goes further: Deborah Cornelius is attractively rebellious as Clara Eynsford-Hill and Matthew Whittle ingratiatingly smiley as her brother Freddy. Very enjoyable.

It's rare for a new play in the West End to make it to a third cast. It's unheard of for a new play in the West End to make it to a third cast with performances that match the ones given on the opening night. In Art, Henry Goodman, Roger Allam and Stanley Townsend have taken over from Anton Lesser, David Haig and Mark Williams, who took over from Albert Finney, Tom Courtenay and Ken Stott. After seeing each trio I'm sure Art will become a classic, played wherever three actors are in search of a decent role. Each cast finds its own highs and lows, presents the characters in a new light, and shifts the balance and tone of the argument, so that you wonder whether Yasmina Reza (and translator Christopher Hampton) haven't added in a bit here and there, as lines leap out with unexpected force. Each cast tilts this supremely poised and skilful play in a new direction. And each time it works.

Yet again, director Matthew Warchus orchestrates three funny and blistering performances. (His other achievement is to provide a taut, stylised momentum to the scenes.) This is the snappiest and most self-doubting of the three Arts. Henry Goodman's Marc has a desperate, biting quality - as aware of his own failings as he is of others'. Roger Allam has a punctured, plaintive charm as the art-loving modernist Serge. While, as the lachrymose Yvan, the towering Stanley Townsend does the wedding- invitation speech with showstopping gusto. If you haven't seen Art, and thought that 10 months on it might have gone off, don't worry: it absolutely has not.

The young playwright Martin McDonagh has four plays set in the west of Ireland on in London and I would certainly recommend you see one of them. I wouldn't recommend you saw more than one. The stories may differ, but the appeal is the same. His best is the first: The Beauty Queen of Leenane. This is now joined by two other four-handers - A Skull in Connemara and Lonesome West - in The Leenane Trilogy at the Royal Court. (The Cripple of Inishmaan plays at the National.)

The trouble about watching three McDonagh plays in a day - or, indeed, four within a year - is that we get wise to the underlying patterns in his work: the rhythms of the threats and insults, the faux-naif literalism of the dialogue, the "fecking this" and "fecking that", not to mention the casual cruelty he metes out to his characters. The central tension in McDonagh's plays lies between the unreliability of any information and the characters' comic fastidiousness about establishing facts. Quite quickly we learn that anything we are told about anyone may well turn out to be untrue. McDonagh has tremendous facility for creating conflict. But the thick ink with which he sketches in his characters puts me in mind of a Giles cartoon. Only in Beauty Queen does he succeed in creating people that you feel confident in caring about: which is not enough to sustain a whole day out.

Back to Kiss Me Kate in Regent's Park, which had its press night rained off last week. Cole Porter's backstage musical about a company touring The Taming of the Shrew receives an engagingly colourful production from director Ian Talbot. Louise Gold is outstanding as Lili Vanessi, the actress who plays Katherine: as good at the mock-operatic bits as the fierce indictment of men. Andrew C Wadsworth is witty and debonair as the director and Issy van Randwyck plays Lois as a hip-wiggling Forties floozie who should be entertaining the troops. Definitely worth a visit - so long as it isn't raining.

'Pygmalion': Albery, WC2 (0171 369 1730). 'Art': Wyndhams, WC2 (0171 369 1736). 'The Leenane Trilogy': Royal Court, WC2 (0171 565 5000). 'Kiss Me Kate': Open Air, Regent's Park, NW1 (0171 486 2431).