Sherlock fans’ obsession with the filming of the BBC detective drama has led directly to changes in the way the show is scripted and shot, says co-creator and star Mark Gatiss.

The phenomenon, known as #Setlock, sees groups of fans gathering together in their hundreds at shooting locations, using the Twitter hashtag to share photos of stars including Benedict Cumberbatch, Martin Freeman and Gatiss himself, as well as information they have gleaned.

And while Gatiss says he doesn't resent fans' enthusiasm, he admits that after three series it is now something that has to be taken into account during production.

"When we were filming Baker Street exteriors last time, the fact you’ve got about 300 people behind crash barriers is… interesting," Gatiss tells RadioTimes.com. "[So] we have factored in trying to minimise large scenes outside.

"If you’re just drawing up in a taxi and running through a door, it’s easier but large dialogue scenes outside are quite tough."

Along with the risk of potential spoilers, one of the main problems is the difficulty the actors face when performing in what Gatiss says amounts to "a live studio audience" situation.

"First of all it gives a lot away, which is a shame, but also just in terms of concentration it’s hard when you feel like you’re being observed by more than just the crew,” says the star, who plays Sherlock's brother Mycroft.

A further complication of having so many members of the public in the vicinity of a shoot is that it can sometimes be impossible to avoid them turning up in the background.

Eagle-eyed viewers watching series three opener The Empty Hearse spotted a group of Sherlock fans in shot in a scene filmed at a London tube station.

Meanwhile, the most famous Sherlock moment so far – the detective's plunge from the roof of St Bart's Hospital – was filmed multiple times, with various red herrings thrown in, in order to avoid the real solution being leaked.

Gatiss and series co-creator Steven Moffat even made tongue-in-cheek reference to fans' obsession by including the fake "solutions" to Sherlock's death-defying escapade in the episode.

Gatiss and Moffat have always been at pains to point out that they avoid reading any of the reams of fan fiction that has sprung up around the show – but one way or another, there's no denying Sherlock fans are having a direct impact on the direction the series takes.

Mark Gatiss supports the London Lesbian and Gay Switchboard, which is celebrating its 40th anniversary this year


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