The Malcontent - Swan - Stratford-Upon-Avon. The Golden Ass - Shakespeare's Globe - London. The Associate - NT Loft - London

Lovely verse-speaking for a beatnik sewer rat

Antony Sher is giving the high and mighty an earful. It's his new vocation to scurry round with his teeth bared in a snarling grin, telling those in power that they're monstrous villains, smarmballs and suckers. This is not, I hasten to add, backstage at the RSC where wrangling has apparently abated since the announcement that the boss, Adrian Noble, will be replaced next Spring by associate director Michael Boyd.

No, Sher is playing Altofronto, the deposed Duke of Genoa, who's incognito as a kind of railing jester called Malevole in The Malcontent. That's John Marston's Machiavellian palace drama of 1603, which just preceded the death of Elizabeth I. Dominic Cooke directs this challenging play with clarity and vigour to complete the Swan's bold season of forgotten Jacobean works (overseen by Greg Doran). So the RSC isn't simply the Really Scuppered Company.

Cooke updates the action to a 1970's Latin American military dictatorship. The mood is set by sleazy, cacophonous jazz and the distant sounds of helicopters and gunfire, while Robert Innes Hopkins' stage design is winningly simple ­ one slow-revolving ceiling fan, red drapes hinting at theatrical pretenders, and high wooden shutters leading to a glimpsed grand balcony. Certainly, Marston's ferocious damnation of political corruption seems to be timeless as Duke Pietro's followers parade in deceptively spotless white uniforms, yanking out handfuls of cash for contract killings on the side. Pietro's right-hand man, Mendoza (Joe Dixon), plots a take-over, wrongly presuming that Malevole will do his dirty work then handily "disappear" ­ along with any duchesses seduced en route.

If The Malcontent is a rediscovered gem, it's definitely a rough diamond. Key plot twists feel rushed, particularly the switch by Pietro's wife (slinky Amanda Drew) from shameless adulteress to contrite penitent. Dixon and Colin McCormack, who portrays the gullible Pietro, tend to look like flat-pack versions of Iago and Othello when embroiled in cunning machinations and cuckolded rages. More generally, Marston's writing can be obscure and convoluted (even after surreptitious cuts).

However, that is offset by some terrifically earthy lingo (as in the memorable exchange: "How did you kill him?"/ "Slatted his brains out and soused him in the briny sea"). Malevole's rants ­ putting one in mind of Thersites in Troilus and Cressida ­ are radically fierce and funny as well.

And although Sher irksomely hams up Altofronto's loving and patient side, he is on fine comic form as Malevole ­ looking like a beatnik sewer rat with greasy locks, beady eyes behind busted specs, and a disgracefully soiled vest. His verse-speaking is crystal clear and wittily injected with modern, sneering intonations, and his character's ambiguous blend of malign vengeance and Christian reforming instincts keeps you on the hop. At its best, scathing entertainment.

You'd have to be Eeyore not to enjoy the barmy fun of The Golden Ass, though it really is way too long. It's refreshing to find a playwright who's alive and kicking, jazzing up a classic text at Shakespeare's gaff. And Peter Oswald's new dramatisation of Lucius Apuleius's first-century, Latin picaresque novel also has interesting connections with Bottom in A Midsummer Night's Dream (playing in rep) ­ not to mention Love's Labour's Lost and Faustus, Boccaccio and Cervantes.

Our itinerant and fancy-prone hero, Lucius (Mark Rylance), pictures himself as a noble scholar but is distracted by sexy ladies and magic. Naively meddling with a witch's potions, he is transmogrified into a beast of burden which leads to years of hard graft before the goddess Isis graciously passes by and points him towards a sacred procession heading for the sea shore. There his bestial trappings fall away at last.

Director Tim Carroll offers lashings of midsummer silliness ­ much of it on wheels and in jokey Edwardian costumes. Some of this is ebulliently inventive, including Rylance's first appearance symbolically riding on a steed (idly rolling in on a cane chair that's the back end of a well-groomed butler). Nevertheless, Apuleius's episodic ramblings can get tiresomely garbled and the ensemble's panto-style antics outstay their welcome.

Mercifully, Oswald's stylistic morphing keeps re-engaging your attention. Flights of lyrical rhetoric knowingly tumble into comic nonsense and there are extraordinary changes in key and genre after the initial larks. In the midst of Lucius's run-in with bandits, an old woman starts telling the tragic love story of Cupid and the over-curious Psyche and they materialise from a cauldron as haunting, delicate, gold-leafed puppets. Lucius, as an ass, suffers far more seriously than Bottom and a social message about maltreatment of the "lower orders" come across strongly. Though Rylance milks some speeches to push the show towards profundity, this actor's forte is playing fools with unexpected poignancy. His Lucius ­ transformed with buck-teeth, a donkey jacket and steel-capped boots ­ brays like a howling human being and, in the end, is almost too spiritually broken to be divinely restored to his former self.

Maybe The Associate is meant to be about transformations too ­ especially moral dilapidation. In Simon Bent's new semi-comic crime thriller, Ray and his mate Tiny are redecorating a council flat. A patch of wallpaper has been stripped and ladders and paint pots are littered around ready for employment. Only the lads are taking a tea break ­ actually, a full roast dinner break at lights-up. The nosh has been generously cooked by the old tenant Mr Watson, whom, we gather, used to run a sweet shop and loves his Bible.

But Mr Watson has a Mr Hyde side. His views on society are sour ­ sometimes bonkers ­ and the audience soon glean he's a lone terrorist who bombs supermarkets. Ray seems affable to begin with, explaining he's a veggie and anti-vigilante. In the end, though, it's dog eat dog as Ray smells money.

Unfortunately, Bent's play plummets ­ just like his characters ­ in one's estimation. What starts off intriguingly cranky ends up desperately creaky.

The opening scene's would-be intellectual dinner-party chat is heartily satirical and surreal. Thereafter, you think Bent probably made Ray and Tiny dimwits because he can't construct a crafty detective yarn. You'd have to be thick as two short planks plus a stack of chipboard not to suss what Watson is up to within minutes ­ with a cache of fireworks, butane gas canisters and old alarm clocks in his garage.

Beyond that, Bent's handling of terrorism seems almost offensively shallow. His comedy is superficially applied, with none of the bite of Martin McDonagh's send-up of Irish extremists in The Lieutenant of Inishmore. What's more, you never get into the mind of Watson, whom John Normington plays with bland cool and the odd melodramatic darting glance. Matthew Rhys's Tiny is a very funny super-slacker and Nicholas Tennant's Ray is alarming when his chumminess peels away to reveal brooding violence. Too often, though, director Paul Miller lets the pace slacken. All in all, a damp squib.

k.bassett@independent.co.uk