In the stalls at the Apollo on the first night of Sylvia, a new comedy by the American boulevard playwright AR Gurney, two of Britain's more seasoned playwrights - Peter Nichols and Keith Waterhouse - were sitting, respectively, in the row in front and the row behind me. After the first five minutes I could imagine them going: "Damn, wish I'd thought of that."

For Sylvia opens with a neat theatrical conceit: middle-aged financier (Robin Ellis), living in Manhattan apartment, finds stray dog and brings her home. The twist is the dog is played by Zoe Wanamaker, who scampers round the room, sniffs the furniture, climbs over the cushions and shits behind the sofa. That's not in itself particularly funny. What is, is the way the financier talks to the dog and the dog talks back. One of the great platonic relationships has found its way on to the stage.

In Michael Blakemore's polished production, the casting is good too: many actresses can play bitches, but few are as well suited to play this sort as Wanamaker. She has the doe-eyes, the pixie face and the endearing ski-jump nose. When she nuzzles and wiggles and stares imploringly you want to throw her a bone.

Not least because she hasn't got much else to get her teeth into. Husband wants the dog. Wife doesn't. Marriage suffers. The elegantly disdainful Maria Aitken, who somewhat improbably teaches Shakespeare to inner-city schoolkids, has the thankless role of the complaining wife. Sylvia to her is Saliva. It was a risky joke for the playwright to have made.

Having put a fresh spin on the conventional triangular relationship, Gurney lapses into sketch-like scenes: trip to Central Park, late-night walk, showdown with the wife, and - yes, this is Manhattan - a trip for the three of them to the marriage-guidance counsellor. Along the way, Gurney touches on mid-life crises, the dog as wife-substitute and urban man's need to get back in touch with nature. But he shows considerably less interest in these themes than Wanamaker shows sniffing round the lamp-post ("just picking up my messages"). We readily invest Wanamaker's Sylvia with human qualities, but when Wanamaker returns from the vet after being spayed - no joking matter, we imagine, for a dog - the scene is played for its winsome comedy. This complacency has more bark than bite.

The playwright Peter Whelan is establishing himself as our leading exponent of speculative - or "what if" - drama. In School of Night he dramatised the mysterious circumstances surrounding Christopher Marlowe's death in 1593. In Divine Right, which opened last month in Birmingham, he made an 18-year-old Prince William decide whether in the year 2000 he wanted to be King or not. (Answer: Not.) In The Herbal Bed, appropriately enough at Stratford, he resurrects a case involving Shakespeare's elder daughter, Susanna.

There are two Whelans at work in these speculative plays. The first is a tough private eye who constructs crafty old-fashioned thrillers. The second is a schoolmaster who raises the tone of the proceedings with lofty themes and explanations. I know which one I prefer.

In The Herbal Bed, Whelan builds a remarkably convincing drama about puritanism and marital infidelity from only a few pieces of evidence. The central characters are not as intriguing as their situation. Shakespeare's daughter, Susanna (the slightly vapid Teresa Banham), is married to a Stratford physician, John Hall (played with brisk importance by Liam Callaghan). In 1613 Susanna is publicly accused by a reckless young gentleman, Jack Lane (raffish David Tennant), of having been "naught" with a local haberdasher Rafe Smith (an earnestly infatuated Joseph Fiennes). Despite the fact that Lane recants, a charge of defamation is brought at the diocesan court at Worcester Cathedral. Cue courtroom drama.

Whelan's masterstroke has been to make the allegations neither entirely true or entirely false. Susanna can survive with her dignity intact if she is economical with the truth. She tells the slow-witted Fiennes: "It's only a matter of what you leave out."

The trouble is that the tight-lipped Vicar-General, Stephen Boxer, wants to hear all the facts. Boxer has great fun, menacingly sifting through documents and delivering dusty rebukes. Elsewhere, in Michael Attenborough's enjoyably taut production, there's a gemlike performance from Jay McInnes, the diminutive wide-eyed maid, around whom the whole cross-examination collapses. I've never seen anyone carry bedding across stage with a greater sense of purpose.

There's a moment towards the end of Tim Albery's production of Macbeth, also at Stratford, when Seyward is told of his son's death. Traditionally a moment of grief, this time it's a matter of callous disregard. The lights switch abruptly, and instead of Macduff entering with Macbeth's head, we return to Macbeth's corpse downstage, and Malcolm delivers his final speech (traditionally a rallying one) seated apart on a bench, full of uncertainty. Till the bitter end, Albery's production stays self- consciously different.

Perhaps taking his cue from Harley Granville-Barker's preface (reprinted in the programme) which describes the play as "cold and harsh and unrelenting", Albery, and his designer Stewart Laing, come up with the chilliest and most inhospitable of the recent rash of Macbeths. Everything is unusual. As the thane, the excellent Roger Allam, who looks as if he might be in charge of a Russian U-Boat, shifts between intimate charm and intemperate rage. His wife, Brid Brennan, enters reading the letter as if she's lecturing a primary school. From there on, she becomes increasingly stricken and strident. The interior of the castle looks like a Scandinavian motel. The ugly mix of costumes, lighting cues and distracting set means we never imaginatively enter this world. We only observe it. Philip Quast, however, is a impressively intelligent Banquo.

Sarah Kane follows up her famously controversial Blasted with Phaedra's Love, her own version of the story that exercised Euripides and Seneca. Her Hippolytus is a 1990s king watching TV, masturbating, eating burgers and getting blow jobs from Phaedra and the priest. When Hippolytus's severed penis is chucked across the tiny Gate Theatre and then placed on a barbecue I was reminded of another old story. One about an emperor and his new clothes.

Theatre details: Going Out, page 14