Hamlet, Old Vic, London<br></br>Coyote on a Fence, Duchess, London<br></br>The Dog in the Manger, Swan, Stratford-Upon-Avon

The idea is admirably risky: Trevor Nunn has cast a complete unknown as his West End Hamlet. Ben Whishaw has been catapulted from a bit-part in the National's His Dark Materials to star at the Old Vic where previous Princes of Denmark have included Gielgud, Olivier and Richard Burton.

Nunn's adventurousness goes further, too, for his Ophelia is a 19-year-old undergrad and Horatio is fresh out of drama school - along with several other company members. Nunn says his inspiration has been Shakespeare's own reiteration of the words "youth" and "young" in Hamlet. His production is also in modern dress, so Jotham Annan's Horatio turns up from Wittenberg Uni in designer trainers and a hooded sweatshirt. Tom Mannion is a white-suited, presidential Claudius whose gun-wielding guards vaguely suggest a military dictatorship. The climactic duel is a fierce fencing match, courtesy of a sports coach's kit bag. Meanwhile, Samantha Whittaker's Ophelia tells her father she has been sewing in her closet but we have seen her dancing around with a ghetto blaster.

Sometimes the immaturity of Nunn's actors results in a freshness and authenticity that is electrifying. Whishaw raises a laugh as the embodiment of stroppy adolescence, moping in a black woolly hat at Gertrude and Claudius' drinks party. At the same time he is intensely distressed, hardly able to speak his first soliloquy for sobbing. That actually makes great emotional sense for "O that this too too sullied flesh would melt" is a speech syntactically riven by grief.

With Whishaw's cadaverously skinny and geeky princeling, it's almost a case of "Frailty, thy name is Hamlet". He is verging on a mental breakdown, neurotically twisting his hands and shuffling like an old tramp, derelict before his time. Simultaneously, you catch touching glimpses of his younger undamaged self, when he flings his arms round his college friends or momentarily curls up on his mother's lap.

The trouble is that this performance is hit-and-miss. Whishaw rushes through certain lines. Moreover, in spite of his advice to the travelling players, his gestures can look self-consciously appended to his words. Nunn is normally better at naturalism. Even some of his experienced actors are showily demonstrative here. Imogen Stubbs charts Gertrude's tragedy strikingly, progressing from thoughtless contentment to harassed alcoholism, but she overdoes her tactile attachment to Mannion. Ironically for a tragedy centring round an inactive hero, one wishes the director would tell his troupe to do less.

Coyote on a Fence is another curate's egg. Bruce Graham's American Death Row drama - starring Ben Cross (of Chariots of Fire fame) - is based on the real Texan case of James Beathard who published positive obituaries about his fellow inmates. Graham corresponded with Beathard and this four-hander tackles moral questions surrounding the death penalty as John - the obit-writing character - strains to respond sympathetically to his cell-partner, Bobby, a redneck racist headcase.

Sarah Esdaile's production, transferring from Manchester's Royal Exchange Studio, boasts strong performances. Cross's John is dignified, ethically questionable and occasionally menacing, while Eric Loren seems nice but shifty as the visiting journalist. Jo Martin is superb as the slouching, tough prison guard with a troubled conscience. But Alex Ferns's Bobby feels like a theatrical fiction. A comically bouncy, cute simpleton with monstrous rabid moments, perhaps he is meant to be ethically provocative. It doesn't work. All in all, there are few amazing insights and the tensions just simmer.

Leaping off the fence now, The Dog in the Manger is an absolute delight. This little-known gem by the prolific 17th-century playwright Lope de Vega gets the RSC's Spanish Golden Age season off to a scintillating start, translated with zest by David Johnston and staged by Laurence Boswell. Designer Es Devlin has transformed the Swan into a gold chamber with a gleaming metal floor which is sumptuous and sunny but also cold, hard and sinister. That perfectly fits this edgy romantic comedy about a virgin-countess, Diana, who adores her lowly steward Teodoro but keeps refusing to marry him. De Vega's class-crossing lovers are fascinatingly akin to those in Twelfth Night and All's Well..., and the switches between boisterous comedy, satire, suspense and near-tragedy are breathtaking. Boswell's ensemble are superb. Joseph Millson's beautiful Teodora is desperately confused and simultaneously a hilarious cad - ditching his first love, Marcela, and suggesting Diana might like to subject him to a bed trick. Simon Trinder, as his serving man, is irrepressibly impish. Claire Cox's Marcela is poignantly tearful, and Rebecca Johnson's jealous rages, as the Countess, are all the funnier for her corseted, exquisite finery.

There are a few moments where you want to turn the volume down slightly and the happy ending feels hasty. However, Boswell cleverly tightens that up, creating something more like Shakespeare's problem plays by having the couples finish with a partner-swapping dance full of bitter-sweet ambivalence. See this.

'Hamlet': Old Vic, London SE1 (020 7369 1722), to 31 July; 'Coyote on a Fence': Duchess, London WC2 (020 7494 5075), booking to 17 July; 'The Dog in the Manger': Swan, Stratford-upon-Avon (0870 609 1110), to 2 Oct

k.bassett@independent.co.uk