After making reading hip again - the books have sold more than 400 millions copies around the world - it was inevitable that films were set to follow, and in 1999, author J.K. Rowling sold the film rights to the first four books to Warner Bros.

Following Rowling's edict that the principal cast must be British, child actor Daniel Radcliffe was picked to portray the boy wizard. Radcliffe had previously appeared in a BBC production of David Copperfield, and his film debut, The Tailor of Panama, was released just eight months before the first film, Harry Potter and the Philosopher's Stone.

The roles of Harry's best friends, Ron Weasley and Hermione Granger, were given to unknowns Rupert Grint and Emma Watson respectively: Grint has gone on to star in several offbeat movies, while Watson has voiced a major Hollywood cartoon and been named the new face of Burberry.

But it is for their faithful representations of their spell-casting characters that the trio will go down in history.

As readers of the novels grew up with the characters, so have the actors. In the first film, they were very obviously inexperienced children playing children. Their acting was inconsistent, at times nervous, which clashed with the ambitious special effects and the stellar adult cast, which included Dame Maggie Smith, Alan Rickman, Robbie Coltrane and John Cleese.

But in recent films, the young stars have come into their own, maturing alongside their characters, with director David Yates, saying the three were 'really keen to push and be pushed in their roles because we want these characters to grow and develop both with the unfurling stories and the audience.'

This maturity is especially important in the film of the sixth instalment, Harry Potter and the Half-Blood Prince, where the evil Lord Voldemort is threatening to destroy both the wizarding and Muggle, or human, worlds, and where teenage hormones are running rife.

As well as a final showdown with Voldemort, the characters must deal with their increasingly confusing feelings towards one another: Harry finds he is falling for Ron's younger sister Ginny, Ron starts dating the rather silly Lavender Brown, and Hermione, who's always held a torch for her flame-haired friend, goes out with jock Cormac McLaggen.

As in any teenager's last years at school, Harry's sixth year at Hogwarts School of Witchcraft and Wizardry promises to be action-packed and full of surprises. And if you don't know, or can't remember who the Half-Blood Prince is, hold on - the film opens on Wednesday.