THEATRE / 25 under 35: It's the role for actresses of that uncertain age. Today, Imogen Stubbs. Tomorrow . . ? Georgina Brown plays casting director

There are a handful of big classical roles for women - Rosalind, Hedda, Cleopatra and St Joan among them - exciting and exacting parts that present an actress with a chance to prove herself to be extraordinary, a chance indeed to stake her claim as an heir to greatness. St Joan, a glorious part in dire danger of being smothered in a flood of other people's words, is perhaps the trickiest of all. An actress needs infinite variety; she must be boyish, brusque, inspired, exalted, mannerless, tactless, victimised and victorious. The best - Eileen Atkins, Judi Dench, Frances de la Tour, Joan Plowright - have made gold of the material. Last week it was Imogen Stubbs's turn.

Stubbs belongs to the generation of actresses who stand in the shadow of an awesomely accomplished bunch - Juliet Stevenson, Fiona Shaw, Harriet Walter, Lindsay Duncan, Frances Barber, Imelda Staunton, Cherie Lunghi, Cheryl Campbell, Brid Brennan, Miranda Richardson, Emma Thompson, Clare Higgins, Phoebe Nicholls, Penny Downie - each one as comfortable in the classics as in contemporary stuff, at ease on stage and screen. But these are the 35-and-overs, as glamorous as they want to be, but decidely over the hill for St Joan. (Blood is squeezed from stones more readily than the age of a thirtysomething actress is extracted from her agent. 'We don't even discuss age. X can play any age. No, I couldn't possibly ask her,' they invariably snap. One agent sustains the mystery by omitting from the biographies the year in which a role was played.)

In many ways Stubbs' intense, physical, tomboy energy made her an natural choice for the part. Moreover, like so many actresses in their thirties, she has not only played supporting roles in classics for the RSC but come to wider public attention through television (in her case, as the girly detective Anna Lee), and is ripe for something seriously centre-stage and beefy. But some very strong contenders must also have made it on to the casting director's short-list, not just for this play but for any classic.

First of all is the gang of seasoned thoroughbred Shakespearians, gutsy actors who are assured verse speakers - Samantha Bond, Joanne Pearce, Amanda Root, Josette Simon, Claire Benedict, Sophie Thompson, Sylvestra le Touzel and Saskia Reeves. Janet McTeer (stunning in Matthew Warchus's Much Ado) is up there with them but she - like Sally Dexter - has a voluptuousness that seems to alarm the less audacious casting directors.

By contrast, the skinny yet anything but slight Jane Horrocks falls into a different slot: she has a more modern look that might explain why she has yet to be cast in a major role in a classic. ('And I wouldn't want to see her in it if she was,' said one who rates her talent but believes her range might be limited.) It might simply be that she's been far too busy.

The next division is harder to pigeon-hole. Lesley Sharp (another actress whose performance in a Mike Leigh film had the bizarre effect of making you believe, quite mistakenly, that you know her) has that useful ability to look lovely or genuinely plain (not like Julia Roberts slumming it) which has enabled her to work on both classics and contemporary stuff. Which is more than can be said for Natasha Richardson and Joanne Whalley Kilmer (and Helena Bonham-Carter, for that matter), first-rate actresses who can be relied upon to put more bums on seats than most, but are hampered by being incapable of looking anything but glamorous. It's this quality, perhaps, that lends itself so well to movies.

Cast the net a little wider, to those with less experience but enormous promise, and such names and faces surface as Beatie Edney, dead funny as the busty bimbo in Terry Johnson's Dead Funny; Clare Holman, whose splendid forthright quality is used to great effect in Millennium Approaches and Perestroika at the National; Helen Schlesinger, an actress with a clever, characterful face and a powerful presence (excellent in The Mill on the Floss for Shared Experience); Claire Skinner, who was bliss as Cicely in Nick Hytner's The Importance of Being Ernest; Sandy McDade, eccentrically gawky, skinny and Scottish and immediately striking (remember The Life of Stuff); and Siobhan Redmond, another Scot, much easier to cast for being more conventionally pretty and willowy. It would be fascinating to see how far these actresses could go, given the challenge of a line of parts for the RSC.

Then there are those actresses so good at belting out a song, dancing up a storm and flashing a radiant smile that it makes it almost impossible to imagine them in parts with a complex internal life. Joanna Riding, for example, was perfect in Carousel, so sweet and affecting that she is surely our next Julie Andrews. So too Ruthie Henshall (everyone went crazy for her in Crazy for You and they are loving her in She Loves Me) and Janie Dee, who has lots of bright-eyed charm and sex appeal (Johnnie on the Spot, National), if not quite so much as Kelly Hunter, who is very sexy, very bold, very talented (Blue Angel).

But at their heels there are some prodigious, precocious talents, many of whom have been snapped up by the National and RSC. Helen McCrory is flying high as Nina in The Seagull at the National; Rachel Power was beguiling in the telly Middlemarch and is superb as the prickly Masha in The Seagull; Susan Lynch is a bold and beautiful Ximena in Le Cid; and Anastasia Hille, recently a gorgeous and formidable Lady Macbeth. At the RSC Sarah Woodward is the resident English rose (a younger Samantha Bond) currently playing Miranda in Sam Mendes' Tempest (if Simon Russell Beale was just a tweak less mesmerising, the critics might have given her more attention); Debra Gillett is outrageously funny in The Country Wife; Alexandra Gilbreath has tiny parts in The Country Wife and Ghosts, but such charisma that the audience can't take their eyes off her; and Emma Fielding, who was utterly compelling as the very young Thomasina in Arcadia (for the National), has been lined up for almost everything in the RSC next season.

Elsewhere, talent waits in the wings, peeping out occasionally to remind you how spoilt for choice a casting director must be. Helen Baxendale, whose work at the Citz won her the Ian Charleson award last year, shines out in Cardiac Arrest; Juliet Aubrey, impressive (despite rotten carriage) as Dorothea in the telly Middlemarch, needs a stage outing; Marianne Jean-Baptiste, superb as Marianna in Cheek By Jowl's Measure for Measure, is worth watching out for. And in a desperately overcrowded market two relatively untried actresses appear poised for the big-time. Rachel Weisz and Emily Mortimer (John's daughter) are both arrestingly pretty and brainy with it (at Cambridge and Oxford respectively). Weisz has her own theatre company and a part in Design for Living this autumn; while Mortimer, fresh from finals, is already filming. But as one actress says sagely: 'In the acting business there is only one golden rule. Never look over your shoulder.'

(Photographs omitted)