Rachel McAdams, Robert Downey Jr., and Kelly Reilly Courtesy of Warner Bros.

Sherlock Holmes star Robert Downey Jr., director Guy Ritchie, and producers Lionel Wigram and Joel Silver were joined by Rachel McAdams, Mark Strong (Body Of Lies), and Kelly Reilly (Mrs. Henderson Presents) at the Freemasons' Hall in London to announce the beginning of production on their highly anticipated flick. The big reveals were that Downey Jr. will be joined by Jude Law as Dr. Watson, Reilly as Watson's love interest Mary Morstan, Strong as the villain Blackwood, and McAdams as Irene Adler, a wily female criminal described by creator Sir Arthur Conan Doyle as "the only woman who ever bested Holmes." (Jude Law had a previous engagement at his child's school and wasn't available.) All involved made it clear that Ritchie's version is going to be a sexier, more rousing, and action-stacked take on the author's legendary detective. So, the latest big-budget studio reboot that will try to haul a beloved literary character into 21st-century franchise-ville...

Law's casting, in particular, steers Watson away from the fusty, bumbling, middle-aged character we're used to seeing in previous Holmes' adaptations, turning him into a youthful ally for the great British detective. "He's an army surgeon, a veteran — he is somewhat of a badass himself," says Downey Jr., speaking on behalf of his absent sidekick. "It's one of those codependent relationships where he's trying to ground Holmes, who is flighty and very much living in his head and in the case. But, as in the stories, he has this yearning to have a real life with Mary Marstan, which makes for conflict with Holmes. We've agreed that we really want to get elements of that Butch and Sundance thing into it."

Several details were kept firmly under wraps, including whether another villainous adversary is yet to be announced (Holmes' legendary nemesis Professor Moriarty has been mentioned in Web rumors) to back up Blackwood and Adler. (Blackwood doesn't even feature in Conan Doyle's books, with Downey Jr. calling him "an amalgam [of different villains] and an appropriate first person for Holmes to match wits with".) Everyone kept tight-lipped about the possibility of a further big casting announcement, with Downey Jr. grinning widely and producer Wigram declaring, "Rather not say at the moment." We'll take that as a yes.

Ritchie and Silver also declined to reveal whether Downey Jr. will be donning Holmes' traditional deerstalker or tweed, but as they've picked 1891 as the date to set their story, it's unlikely he'll be adopting a leather or spandex look. Judging by Downey Jr.'s description of the 19th century crimebuster, though, don't count anything out: "The more I look into the books, the more fantastic it is. He's just such a weirdo. As a matter of fact, Mrs. Downey [Downey Jr.'s wife Susan, a producer on the film] said that when you read the description of the guy — quirky and kind of nuts — it could be a description of me." His lethal fightings skills (bare-knuckle boxing, jujitsu-style martial arts) and cerebral, crime-solving abilities will all be sourced straight from Conan Doyle's canon, as will the plot, which is likely to draw on elements from several of the Holmes adventures.

Shooting kicks off in London and Liverpool on Monday, with Ritchie revealing that St. Pauls Cathedral will be a key location. "I'd like to bring something new to this project," says Ritchie. "I have a few tricks up my sleeve."