Guy's new guise for a master of reinvention

The trailer for Sherlock Holmes, the imminent "reboot" of Sir Arthur Conan Doyle's immortal detective stories with Robert Downey Jr and Jude Law, has excited predictable outrage among fans of the old-fashioned art of detection. Holmes stripped to the waist, belabouring adversaries like Jackie Chan? Holmes leaping into the Thames from the window of his office in the Houses of Parliament? Sir Arthur, say the purists, will be spinning in his grave. But Guy Ritchie's film is far from the first to play fast and loose with the Holmes legend: and the great detective himself, always a master of disguise, has not always been the figure we think we know.

Readers of the stories, for a start, may struggle to recognise in them the figure of the popular imagination: a gentleman in a tweed cape and deerstalker cap, smoking a huge calabash pipe. That famous silhouette was almost wholly the invention of William Gillette, an American playwright, actor and impresario in early 20th-century America, who played Holmes on stage, on film and on radio for three decades.

Conan Doyle once described his detective as taking an "ear-flapped travelling cap" to the country, but Gillette picked the deerstalker out of one or two Sidney Paget illustrations in The Strand Magazine and made it his character's uniform. The enormous calabash was chosen because it was visible from a distance and could be held in the mouth while the hands did something else, two theatrical advantages that the clay pipes and cherrywoods preferred by the printed Holmes conspicuously lacked.

The Ritchie film is also far from being the first revisionist take on the stories. Each generation has re-created its own vision of Holmes, starting in 1908 when a Danish film company began to produce silent adaptations that, equally silently, updated the detective by 20 years to a contemporary setting. This vision of a modern Holmes lasted until the Second World War, when the classic Basil Rathbone films dared to present Doyle's stories in a Victorian setting.

Within two films, however, the timeline had jumped 50 years and Holmes and Watson were sniffing out Nazi spies. Other noteworthy adaptations (there have been more than 200 in all) have involved Holmes investigating the Whitechapel Ripper murders or undergoing treatment by Sigmund Freud, as well as several Soviet-Russian adaptations in the 1970s and 1980s under such snappy titles as The Twentieth Century Approaches.

Ever adaptable, Holmes has even managed to transcend the bounds of fiction. The present-day Sherlock Holmes Society and its various brethren around the world play a game invented in 1928 by Monsignor Ronald Knox, the theologian and detective writer whose satirical essay Studies in the Literature of Sherlock Holmes treated Conan Doyle's inventions as historical characters, purporting to analyse Watson's "manuscripts" on the internal evidence.

As Knox jokingly quoted the fictional critics "Monsieur Piff-Pouff" and "Professor Backnecke" on inconsistencies between the pre- and post-Reichenbach Holmes, he can hardly have imagined the discipline he was creating: writers such as Dorothy L Sayers and generations of hobbyists have since devoted years to teasing out background details from the "evidence" in the stories. Holmes and Watson's literary afterlives, too, have proved some of the most vigorous of any characters'. Conan Doyle's son Adrian began it, collaborating with the detective novelist John Dickson Carr to produce a cash-in volume of Exploits of Sherlock Holmes after his father's death.

Since then, writers such as Stephen King, Stephen Fry, Michael Chabon and Michael Dibdin have taken their cracks at the great detective. A story by Neil Gaiman deserves special mention for casting Holmes as a criminal mastermind, in a Victorian alternative timeline ruled by the pagan gods of HP Lovecraft's Cthulhu Mythos. Purists can comfort themselves with one final thought: Downey's kick-boxing appearance will be far from the strangest thing to happen to Holmes this decade. A forthcoming comedy based on the stories is set to feature Sacha Baron-Cohen (Borat, Brüno) as Holmes, with the Anchorman actor Will Ferrell as Watson. Meanwhile, the Doctor Who writers Mark Gatiss and Steven Moffatt are working on a BBC version of the Holmes franchise set amid the bustle and technology of contemporary London.

This new vision introduces a Dr Watson who has recently been discharged from the British Army ("You have been in Afghanistan, I perceive") and a Holmes who is "virtually a psychopath", according to Gatiss. The faces may be changing, but the casebook of Sherlock Holmes, it seems, is far from closed.