This much is clear: It's 1891, a year after their first adventure, and the great English detective and Dr. Watson are facing off with Professor Moriarty, a mysterious, peripheral character from their initial blockbuster.

Ask the creative forces behind "Sherlock Holmes: A Game of Shadows" for more details on the new Robert Downey Jr.movie, due in theaters Dec. 16, and you'll find tight lips. But the set here, a 40-minute train trip west from London, was rife with clues last winter.

They included two awesomely large German cannons (referred to as Big Berthas), a firefight involving a pair of dubious-looking French twins, and a handful of Gypsies, including a beautiful young gunslinger Sim (Noomi Rapace, of Stieg Larrson's "Millennium trilogy" films).

Jude Law is back as Watson, Guy Ritchie is again directing, and on this particular day on frigid tracks outside of the Didcot train museum, Holmes' head was scuffed up, his handshake bloody.

So what's happening?

"Well clearly," Downey said laughing and gesturing to Law, "he's the one who just rescued me."

Like their Holmes-Watson characters, Downey and Law make complementary interview partners: Law plays the thoughtful professional actor, while Downey is more of a sassy, exuberant ham.

The new film, which is largely told from Watson's perspective, seems to find the doctor tracking down his old friend and trying to keep Holmes from going insane, getting himself killed, or both, as he fixates on decoding another crime.

Watson "sees his strange friend kind of commit to going all the way up to and then over the edge with" Professor Moriarty, Downey explained.

The doctor "realizes that his friend is lost on this path," Law added, as he goes "to the nth degree" in trying to solve the latest elaborate criminal riddle.

If anyone can rein in Holmes, Law and Downey suggested, it may be Watson. In the scene that they were shooting that day, Law said, Watson unexpectedly saves Holmes who, for some reason, isn't the slightest bit surprised. His nonchalance spurs Watson to repeatedly ask: "How did you know that I would come?"

Their complicity, Law added, becomes one of Holmes' "most powerful weapons by the end of the film."

Downey acknowledged that word has leaked out that Sim "is kind of our gal and she is obviously knee-deep in Gypsy comrades" who ally themselves with Holmes and Watson in the battle against Moriarty (Jared Harris). Moriarty, Downey added, is "arguably the first super-villain in the history of literature."

As with the first film, many of the performances are, by design, somewhat eccentric, but in the spirit of author Arthur Conan Doyle's literary vision.

"We haven't gone further away from the book," Law said. "But because it wasn't a visual medium, you couldn't see them apprehending, you couldn't see them escaping." The medium of film, he added, allows for an opportunity to "embellish" the action in the book. Producer Joel Silver, he said, urged the filmmakers to "take the book out of the drawing room."

With Warner Bros. again aiming for a PG-13 rating, the Sherlock Holmes team is constrained a tad by what they can do on screen in terms of creating an authentic level of danger — though Law and Downey said that might be a good thing.

"Sometimes the worst thing you can do is have every toy in the toy box," Law said. "Sometimes it is good to be told: 'Play with those.'"

"Otherwise," Law said, "it can become a free-for-all. The source material was good, so why should our interpretation, however fresh and original to a modern audience, suddenly drift into the world of carnage?"

"The limitation makes you have to be more, ah, considerate," Downey added.

While such creative license might infuriate obsessive Trekkies or Middle Earthers, Downey and Law noted Sherlock Holmes fans are accustomed to myriad interpretations of Conan Doyle's work and are remarkably tolerant.

"We never really had a single person say, 'I can't really look you guys in the eyes, you have desecrated this,'" Downey said. "Not even the 'Baker Street Irregulars,'" a group of Holmes enthusiasts that meets in New York, has complained.

"There's our consultant, Les Klinger, who wrote the definitive annotated Sherlock Holmes book; we'll call him up when we have a question and he'll have concerns, and we will address them. But for the most part, every day when we are shooting and we revamp dialogue, we take Conan Doyle's words," Downey said. "We can't imagine that we are going to beat the way it was originally done. What we are doing is modernizing it and making it cinematic enough, and keeping it interesting enough for us."

With the first film, said Law, people were perplexed by what director Ritchie, known mostly for gritty heist films, would do with the material, as well as how Downey would interpret the role of the legendary English detective. They wondered, Law said, "What is it going to be?"

"He's going to be ab-flashing!" Downey interjected, laughing.

Law continued, unfazed by his partner's shtick: "I think they were really surprised that it was as traditional as it was. They said, 'I get it. It was a twist on something.'"

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