A Masons sub-plot required the film's producers to deal with the notoriously secret group... who turned out to be very open and not at all sinister.

Crashed out on a fold-out sun lounger, Robert Downey Jr. looks tired and scuffed. There's blood on his face and quite a lot of soot on his clothes, generated by a massive explosion that's just occurred out on a purpose-built set on Liverpool docks, which is doubling for 19th century London. Already, Empire gets the impression that in the hands of Guy Ritchie, this latest vehicle for Arthur Conan Doyle's Sherlock Holmes character will not resemble the elegant detective stories that enjoyed cinematic vogue during the '50s. As played by Basil Rathbone he was a gentleman sleuth. As played by Downey Jr. he's frankly more like a raggy-boy, loose-cannon Miss Marple with an unerring nose for trouble.

"I don't want to try to be the intellectual pillar of Liverpool," muses Downey Jr.," but I think Holmes is primarily an archetype, because mostly you only ever hear about people like him, who are such super-specific geniuses. Most people, I'd say, are generalists - they know just enough about a variety of things to not look stupid at a dinner party. But Holmes has gone to the next level, where he's on his own planet with regard to detection. I just feel that it's more of an archetypal thing. It's a pathetic, isolated existence, in that he does it to the exclusion of life, love and all other endeavours. Except a little bit of opera when he's feeling less than inspired..."

To help him through this pitiful existence, Ritchie's reboot has ramped up Holmes' right-hand man: Dr. Watson, played by Jude Law very much as a man's man, and not some fusty, fretting nursemaid.

"When you go back to Conan Doyle's books," says Law, who joins Empire in a big, cold box laughably called The Green Room, "you realise how much is in the gaps, how much hasn't been done yet. What was a real surprise to me was that this is supposedly welltrodden territory - and then you go back and you realise how much has just been taken for granted. I mean, one of my bugbears is that people think of Watson as a doddering old fool, but that comes directly from the Rathbone movies. That's not in the books. Watson's a trained soldier, hot from Afghanistan, where he suffered a war wound. He walks into battle with his revolver as eagerly as Holmes." He grins. "There's really an edge to him."

The story is still shrouded in mystery, although there's talk of Masonic lodges, alchemy and, of course, a likely visit from Holmes' mortal enemy, Professor Moriarty. It also seems likely that the plot will borrow from the whole of Conan Doyle's work (56 short stories and a mere four novels written between 1887 and 1927) rather than one tale in particular. Which explains the appearance of Irene Adler, played by McAdams, who is described as a tantalising mix of adversary and love interest.

"She's described as 'the woman' in the Doyle stories," says the 30 year-old Canadian, "but she's actually only in one - A Scandal In Bohemia - and she doesn't appear again. But she's supposed to be the only woman who's ever bested Holmes, which also leads to a dose of passion. So they're very antagonistic, even though they have a real affection and respect and admiration for one another. They just seem to be on opposite sides of the law, a little bit." She smiles. "Am I going into spoiler territory? Hmm, I may be, so maybe I should stop there ... !"

The constant referencing of the source novels is unusual for a big-budget studio movie, and neither Downey Jr. nor Law can speak highly enough of Conan Doyle. "He was a masterful writer," enthuses Downey Jr.." Now, I hear he lost steam and got a little bored after his salad days with Holmes, but it's all there. It kind of was the first crazy-ass fanboy literature of the 20th century. I think the trippy part is realising that this hasn't been invigorated sooner.

"I remember reading a passage in one of the books," he continues, "where they're working out when Watson should lob a grenade through an adventuress' window - Irene Adler's window, in fact. I'm like, 'These guys are off the hook!' It's set in Victorian London, but I was surprised by how modern it was, and what an enlightened time it seemed to be. Because all we hear is, 'Yeah, the streets smelt like shit,' but everything was kind of like now, really. Everything was about to pop into the next realm, into... well, you could call it the 20th century. But the way it really worked out is that everything was verging on this big expansion of information and technology."

Law nods, before offering the only real clue that either will share about the storyline: "That's in the film, too; the presence of that, and the harnessing of that. That's what their mission is about."

However, the reliance on the original material only goes so far and, perhaps thankfully, Ritchie's Sherlock Holmes won't take the dark-and-gritty approach that restarted the Batman franchise. In particular, there won't be much made of the intravenous drug use - cocaine and morphine - that kept Holmes' mind occupied in the absence of a juicy case to solve. Says Downey Jr.: "All the stuff about the addiction might have been timely in the 1890s when, before the Harrison Narcotic Act, there was a huge upsurge of all that stuff. But Holmes is someone who is pretty much always in control and never a slave to anything. He's just driven. He's a sacrificial archetype, the type of person who does something for the higher good."

He's also a reluctant detective in that, rather like the antihero of A Beautiful Mind, he just can't help solving mysteries. "I like the idea that if he doesn't do that, his powers will somehow turn against him," says Downey Jr.. "But I like the idiosyncratic things, like Watson saying, 'He can be so brilliant, but sometimes he overthinks things.' That's easier to relate to. I have a friend who can sit and debate what movie we should go see at the cineplex until we've missed every last one of them! So, to give you an example, some thugs are coming after us, I'll be fiddling around with the best way to pick a lock, and Watson will just kick the door off its hinges."

If you've seen the trailer, you'll know that there's plenty more of this kind of thing because, above all, Ritchie and co. would like you to be entertained. "It's an adventure, really," says Downey Jr.. "And that's the idea behind it, too. We kept pushing the idea that you have to see this through the good doctor's eyes. Because, like most people, he's comfortable and rooted in his life, and Conan Doyle is speaking to us, through him, saying, 'But don't you want a sense of adventure in your life?' I think that's badass, and PG-13, and there's no compromise made for our demographic, our artistic sensibilities and our process as actors. So we're kinda like pinching ourselves half the time."

And will it be a one-off? The answer from Law is a resounding... "No! The more you work on the substance of the film," he says, "the more you go back to the source material, and the more you realise there is to tell. This relationship went on and on and on, and there are fantastic stories and even better characters. We would love that the audience falls in love with it as much as we have, and then we can tell a whole bunch more."

"We definitely agreed to renegotiate for a part two," confirms Downey Jr.. We feel really strongly about that. Now, I love England, but we might need to shoot the next one abroad."

He grins. "Jude and I'll be texting each other. I'll say, 'Brussels!'; he'll say, 'Gstaad!' We're really gonna dig deep f or the next one."