Measure For Measure, Royal Shakespeare Theatre, Stratford-upon-Avon, ***

Fascist Vienna's flawed fascination

Emma Fielding shot to our attention a decade ago playing a 14-year-old mathematical genius in the early 19th-century sections of Tom Stoppard's Arcadia. Not the easiest of briefs, to put it mildly, but one which Fielding mastered with a haunting spirit and sensitivity. Small, mettlesome, with fiery dark eyes and a fierce wit, she's an actress who can convey, with extraordinary immediacy, a character's passionate belief in ideas and ideals.

As such, she is dream casting for Isabella, the novice nun who, in Measure for Measure, refuses – on principle – to buy her brother's reprieve from execution by sleeping with the acting head of state. So it's no surprise, then, to discover that it's her characterisation that is the beating heart of a pacy and intriguing new revival of the play, set in 1940s Vienna, directed by Sean Holmes on the main stage at Stratford.

"More than our brother is our chastity": the modern mind tends to cast an automatic slur on this position, insinuating that only a selfish, sexually repressed cow would demur at forfeiting so little to achieve so much. But the bony, intense Fielding, in her taut encounters with the false deputy Angelo, thrillingly convinces you of the fervour of this young woman's religious faith. The survival of the body is of far less importance to her than the condition of the immortal soul.

Nor is Fielding's an alienating rectitude. As she denounces the mercilessness of an earthly ruler "dressed in a little brief authority" and contrasts this with the clemency of the Almighty, it is the girl's striking courage, mustered at clearly considerable cost, that floods over the footlights. And it's all the more impressive, given that Fielding also lets you see Isabella's heart-heavy, ordinary human pain at her brother's parlous plight.

If only, in this production, she had an adversary who matched her in force. Daniel Evans is a wonderful actor, but here he has been constrained into a conception of Angelo that is too narrow and pre-emptively derogatory. Evans has been asked to play him as a petty bureaucrat, in suit and rimless specs – the kind of functionary who, like Eichmann, might well have viewed the transportation of Jews to the death camps as primarily a logistical problem well suited to his organisational skills. But, before he is exposed as a hypocrite, Angelo needs to have enough clout to enunciate tellingly the great counter-principle of Justice. To Isabella's plea that he show pity, he counters, "I show it most of all when I show justice;/ For then, I pity those I do not know,/ Which a dismiss'd offence would after gall." Evans's Angelo is not allowed the weight to make those words reverberate troublingly in the mind. He's at his best in the final scene, when it is clear that the play's whole argument about mercy has had no effect on him whatsoever. There's the blackest comedy in his fanatical preference for execution over what is a clearly dreaded marriage to Mariana.

Paul Higgins's nervy, fervid Duke – the eavesdropping arch-manipulator who returns from his supposed sabbatical disguised as a friar – secretly superintends a sleazy Vienna whose streets are filled with black marketeers and heavily pregnant prostitutes who are prepared to slip behind the brick wall that dominates the set. It's a decadent, bewildering, war-scarred world – and one which evidently stirs, in some quarters, a longing for the regimented simplicities of fascism such as Evans's Angelo might well supply.

The one important misinterpretation aside, the production has an admirable freshness, bringing an eye unclouded by theatrical tradition to nearly every turn of the piece.

I loved the hilarious idea of suggesting that the ever-so-upright public executioner Abhorson (David Peart) has the hots for his cherubic new helper, the former bawd Pompey (a very funny Simon Trinder). It's Isabella and Angelo all over again, this time as disreputable farce (and with a rather more compliant victim). A clever additional joke in a flawed, but arrestingly alert account.

To 4 November (0870 609 1110)