Alan Rickman: Beguiling monster who made Cherie weak at the knees

Alternately screen baddie, sex symbol and darling of the chattering classes, he is about to work his considerable magic in the new film version of Harry Potter

Alan Rickman exudes a malcontent sensuality: a jaded, exotic quality that suggests he was destined for a world of opiates and seed-pearl dressing-gowns. It's partly physical: some effect of that gangly body, those narrow eyes and that shaggy mane of hair. But it would still be palpable if you shut your eyes during one of his lavish, expansive performances. His voice is his real secret weapon: low and melancholy, like Robert Stephens without the entropic effects of a diet of gin, and the principal reason why directors call him up if they need a hypnotist, aristo, mad monk, super-villain or – in the case of Harry Potter and the Sorcerer's Stone – a wizard. Which is ironic when you consider that he was brought up on a council estate in Acton, London, and that the languid tone and delivery are the serendipitous result of a speech impediment. Alan Rickman cannot move his jaw properly. It has given him a voice like no other actor.

One consequence is a steady stream of mildly obscene fanmail from middle-class women. "He's now at a crossroads in his career," reflects Maureen Paton, author of the actor's unauthorised biography. "He's immensely choosy about what he accepts, takes forever to decide on a role, and has now decided that he's more drawn to individual projects, and not so keen to do big blockbusters for the exposure." His part in Harry Potter will be the first film villain he's played for ages. As the intensely creepy Professor Severus Snape, he will get the opportunity to introduce a generation of children to the uncanny charisma that their elders have been enjoying for two decades.

Recently, Rickman has been taking stock. His confidence in his own abilities as an actor took a pounding last year, after the miserable experience of playing Antony at the Royal National Theatre. The reviews were mercilessly accurate. Rickman seemed so ill at ease in the role that he was often barely audible, and his trademark long hair seemed ludicrous on the head of a battle-scarred Roman war veteran. "He hated the production," says Maureen Paton. "And so he dug his heels in. If he doesn't like a production, and disapproves of the way it's going, he becomes a master of passive resistance." According to friends, Rickman was so upset that he considered giving theatre up. Success in the West End production of Coward's Private Lives has restored his self-belief.

The Rickman resumé is an inventory of fascinating monsters. On film, he's played Rasputin, Franz Anton Mesmer, the Sheriff of Nottingham, a knobbly-headed alien scientist and – in Die Hard – a power-mad terrorist with a thing about Alexander the Great. Democracy he doesn't do. His most celebrated stage role, as the Comte de Valmont in Christopher Hampton's Les Liaisons Dangereuses, radiated autocratic charm. When he lost the part in the movie version to John Malkovich, he was reportedly furious, but the choice was revealing: both men convey a type of persuasive weirdness, the sort of authoritative effeminacy that might have made Gwendolen Fairfax go weak at the knees.

Even when preposterously over the top, Rickman is engaging. In Robin Hood: Prince of Thieves, he rages with as much restraint as an am-dram Abanazar. ("And cancel Christmas!" is the line everyone remembers). But this amplitude isn't alienating. Instead, it demonstrates that he shares much of the audience's scepticism towards the inanity of the movie. Prince of Thieves was a hit in the UK not because punters wanted to bask in the self-absorption of Kevin Costner, but because they wanted to revel in Rickman's savage amusement at the poverty of the material. There's a conspiratorial energy in such overacting, which whispers to its audience: Let's have fun with this awful movie and then go someplace nice.

Alan Sidney Patrick Rickman made his theatrical début on 12 December 1951, in the Nativity play at his infants' school in Acton. Seven years later, he won a scholarship to Latymer Upper, a private school in Hammersmith, where he transformed his accent to a languorous RP, and established a monopoly on female roles in end-of-term productions. He also gained the lifelong friendship and patronage of his English teacher, Colin Turner, who encouraged him – and other Latymer alumni such as Hugh Grant, Mel Smith and Christopher Guard – to pursue a theatrical career. Turner was a key influence on the formation of Rickman's artistic sensibility. Having lost his father to cancer at the age of eight, the future star responded well to Turner's mentorship. Schoolfriends say elements of Turner's voice and mannerisms are echoed in Rickman's acting style.

He lives in a maisonette in Westbourne Grove, not far from his mother's council house. He met his first serious girlfriend, Rima Horton, in 1965, when they were both students at the Chelsea School of Art. They are still together. Political activism is one shared enthusiasm. Horton is a Labour member of the Royal Borough of Kensington and Chelsea council, and a senior lecturer at Kingston University. (She also stood against Alan Clark in the 1992 general election.) Rickman is a high-profile supporter of the Labour Party – the kind of poster-boy that the urban middle classes adore. Cherie Blair is a fan – and insisted that Tony take her to Private Lives last week on a rare night out. The Folletts and Mo Mowlam are friends, and in 1999, there were rumours that Rickman would stand against Michael Portillo in the election.

Amateur drama was another common pursuit. Rickman spent the late Sixties juggling his commitments to a number of non-professional theatre groups, and accommodating jobs as a graphic designer for a radical freesheet newspaper, The Notting Hill Herald, and at Graphiti, a design company which he helped to found. Then, at the age of 26, he wrote to Rada, requesting an audition. He soon found himself back in full-time education, supporting himself by working as a dresser for Sir Ralph Richardson.

His first engagements were with rep companies in Leicester and Sheffield (where he shared a flat with his staunch friend, Ruby Wax). A stint at the RSC – in which he and Wax added Juliet Stevenson to their gang – saw him miscast and unhappy. Fringe shows and early television roles (in a BBC adaptation of Zola's Thérèse Raquin, and Tinker, Tailor, Soldier, Spy) were more comfortable experiences. The BBC also provided him with his breakthrough role: Obadiah Slope in the television adaptation of Anthony Trollope's Barchester novels. Rickman's Slope was carnal and unctuous, skulking in cathedral architraves like a bipedal eel in a cassock, and stole the series from under the noses of its more established stars.

At 55, he's too old to take the romantic roles he was denied at the RSC. But he's too young to take on the grizzled generals and aged kings. The failure of Antony and Cleopatra has steered him away from Shakespeare. In five years' time, however, he would make a brilliant Prospero. Or he might, if he wakes up at 60 and finds his ambitions evaporated, simply defect to that territory of moneyed oblivion now inhabited by Peter O'Toole, David Warner and other British Hamlets who have learnt to content themselves with cameos in second-rate Hollywood products. Somehow, it's hard to imagine him settling for anything so easy.