“They were coming to me not to put on two stone and fall around and put my foot in the wastepaper basket but wanted a Watson with a bit more edge,” Jude Law says of being offered the role. “He’s been in a brutal war and has experienced horror and physical pain.

“I wanted to represent him as the slightly more buttoned-up, polished professional, with Holmes being the more wayward, eccentric dilettante.

“But he’s far from just bumbling along, he’s in the middle of the action, sometimes tearing in ahead of Holmes.”

Alongside a main plot showing the intrepid pair racing around Victorian London, solving clues and grappling with baddies, there are strong themes of friendship. Holmes is struggling with Watson’s announcement that he is leaving their rooms at 221B Baker Street for his fiancee (played by British actress Kelly Reilly).

“It’s so funny to me because I’m used to people saying ‘you and this female have great chemistry’ but they’re talking about Jude and me like we should be doing romantic comedies together,” laughs Downey Jr, adding that the actors’ friendship also blossomed on the set.

“The second we met, we just started bouncing ideas off each other. We were very much on the same page, which is a pretty eccentric page.

“He really knows what he’s doing and yet he’s very open to letting things flow.”

Eagle-eyed Liverpool viewers will spot Stanley Dock in the movie, doubling as London warehouses next to a CGI-ed Tower Bridge, which was only partly completed in 1890 when the story is set.

“The production design did an amazing job embellishing what were already pretty historic sites, making them beautiful,” says Law, who has filmed in Merseyside before for the 2004 movie, Alfie.

“The locations on this film are a bit amazing.

“I was born and raised here and we went to places I’ve never been or even seen – really beautiful Victorian and Edwardian cobbled corners of London, Manchester and Liverpool.”

The cast and production crew spent their lunchtimes on location in a trailer “making each other laugh” or discussing the afternoon’s work.

Ritchie improved his guitar skills.

“I think there’s something about the work ethic here,” says Downey Jr. “As Americans, there’s sometimes a bit of an abrupt attitude we have – ‘all right, we’re here, let’s get down to the work’.

“We were very shortly put in check that there’s a more civilised way to operate – put out a little cheese, let’s talk . . . ”

He is confident that Sherlock Holmes fans, old and new, will enjoy this new take on the consulting detective: “There’s an esoteric element to this, in that sometimes you just feel like you’re in the right groove. Sometimes you just feel like you are being silently approved of from some other place.”

SHERLOCK HOLMES opens in cinemas nationwide on Boxing Day.

laura.davis