Watson's line to Holmes, "You know that what you're drinking is for eye surgery?", is an obscure reference to Holmes' cocaine usage. At the time, cocaine was used as a topical anesthetic for eye surgery. In the stories, Holmes injects cocaine.
Attempting to escape the clutches of wearing a tight corset every morning, Rachel McAdams would trick the costumers by pushing her stomach out or eating a big breakfast of oatmeal before being laced up. However, they eventually caught on to her ruse.
In the original theatrical release of the film, Professor Moriarty, seen only in shadow, is voiced by an unknown actor. This was done before the official casting of Jared Harris as Moriarty in he film's sequel, Sherlock Holmes: A Game of Shadows (2011). However, since the release of the latter, Harris' voice has been dubbed over the original actor's voice, as to preserve continuity on all new DVD and Blu-ray copies, as well as any televised showings.
Director of Photography Philippe Rousselot used a special high-speed digital camera, specifically to film the Punch Bowl fight sequence. The key moment where Holmes punches his opponent's jaw, was filmed one second in real-time, and turned into a seven-second shot without additional post-production aid.
After examining Lord Blackwood's coffin, the line that Holmes and Watson deliver in unison, "The game's afoot; follow your spirit: and upon this charge, cry--God for Harry! England and Saint George!" is from William Shakespeare's play Henry V.
In the stories, Holmes is described to be an expert in Baritsu. In the film, however, the martial art that Holmes used is the Wing-Chun Kung Fu (famously used by Ip Man Yee and Bruce Lee). Robert Downey, Jr. is a practitioner of the art in real-life, and the fight scene between him and David Garrick at Punch Bowl Pit was coordinated with the help of his trainer.
The film contains numerous references and allusions to the Sir Arthur Conan Doyle novels and stories. Including: "The game is afoot." ("The Abbey Grange", as well as the original source of the phrase, William Shakespeare's "Henry V"); "Because I was looking for it." ("Silver Blaze"); "You have the grand gift of silence, Watson. It makes you quite invaluable as a companion." ("The Man with the Twisted Lip"); "Crime is common, logic is rare." ("The Adventure of the Copper Beeches"); "My mind rebels at stagnation. Give me problems, give me work." (The Sign of the Four); "It makes a considerable difference to me, having someone with me on whom I can thoroughly rely." ("The Boscombe Valley Mystery"); "Data, data, data. I cannot make bricks without clay." ("The Adventure of the Copper Beeches"), "One begins to twist facts to suit theories, instead of theories to suit facts." ("A Scandal in Bohemia"), and "There is nothing more stimulating than a case where everything goes against you." ("The Hound of the Baskervilles").
The film depicts Watson pressing Holmes to meet his fiancée, Mary Morstan. Holmes is reluctant to do so. In the original stories by Sir Arthur Conan Doyle, Mary Morstan is introduced as a client (in the novel "The Sign of Four"). She and Watson fall in love while he assists Holmes in her case, and he proposes to her at the end of the novel.
Although Irene Adler plays a large role in the movie (and works of fiction by "Baker Street irregulars"), she only appears in one Sir Arthur Conan Doyle story, "A Scandal in Bohemia", briefly referenced in the movie. Holmes retains the portrait of Irene Adler acquired for his services in that story and also once refers to her as "the woman" as he does in the latter story.
There are repeated references to five women murdered by Lord Blackwood before the beginning of the movie. Five is also the number of women that the bulk of historians agree were murdered by Jack the Ripper (additional victims are disputed).
The story "The Adventure of the Musgrave Ritual" mentions that Holmes practiced shooting his pistol by putting VR (Victoria Regina) on his wall with bullet holes. In the movie, Holmes shoots VR in the wall in his room with a gun.
When Sherlock says "Now that you're sitting comfortably, I shall begin." to begin explaining Blackwood's plot, it is a reference to a BBC children's radio program from the 1950s, Listen With Mother, which was famous for its opening line "Are you sitting comfortably? Then I'll begin."
The bare-knuckle fight between Holmes and McMurdo references the Holmes story "The Sign of Four". In the story, Holmes encounters McMurdo and says to him, "I don't think you can have forgotten me. Don't you remember that amateur who fought three rounds with you at Alison's rooms on the night of your benefit four years back?"
The four symbols, to which are referred, in the movie, the Man, the Lion, the Ox, and the Eagle, are also attributed to the four Gospels of the Christian Bible: Matthew (Man, the humanity of Christ), Mark (Lion, for courage and for action), Luke (Ox, for strength and perseverance), and John (Eagle, for clarity of sight and for divinity).
The scene in which Holmes and Watson make a series of deductions from a dead man's watch closely mirrors a similar sequence in "The Sign of the Four" (as does Holmes' ability to follow the carriage's path whilst blindfolded), in which Holmes uses nearly identical observations (scratches around the watch's keyhole, pawnbroker's marks on the inside of the case) to deduce information from a watch belonging to Watson's late brother. Holmes's passing reference to locking Watson's checkbook in his desk parallels a similar statement in "The Adventure of the Dancing Men", which commentators such as William S. Baring-Gould have taken to mean that Watson had a gambling problem, an interpretation that the film adopts. Holmes also uses a riding crop as a weapon throughout the film, as he does in "A Case of Identity". In the "Six Napoleons", it is described as his "favorite weapon".
Mary asks Watson if she could read his journals of his adventures with Sherlock Holmes. Of the sixty Sir Arthur Conan Doyle-penned stories of Sherlock Holmes, all but four have Watson serving as the narrator. Of these two are narrated by Holmes ("Blanched Soldier" and "Lion's Mane"), and two by an omniscient narrator ("Marazin Stone" and "His Last Bow.")
The name of Holmes and Watson's English bulldog is Gladstone. He seems to be named after William Gladstone, four-time Prime Minister under Queen Victoria. A Gladstone is also a type of bag, named after the man, which was sometimes used by doctors in this period to carry their medical equipment.
When Holmes (Robert Downey, Jr.) looks back at Irene (Rachel McAdams) when the barrels begin to explode outside the slaughterhouse, the number 221 can be seen on one of the still intact barrels; the same number as Holmes' address (221 Baker Street).
Several of the film's details recall "The Adventure of the Mazarin Stone". The first is the name of the primary antagonist, Lord Blackwood, which parallels that of "Mazarin Stone" villain Count Negretto Sylvius (Negretto is Italian for black, and Sylvius is Latin for woods). (As Holmes scholar W. W. Roberts notes, this is "presumably a private joke at the expense of Blackwood's Magazine, long and unavailingly courted by Sir Arthur Conan Doyle in the 1880s.") Another common detail is the Crown Diamond, an alternate name for the Mazarin Stone, which hangs around Irene Adler's neck in the film. "The Mazarin Stone" is also the first story to mention that the 221B Baker Street apartment had multiple exits and a waiting room. The extra exit, which was through the bedroom, is employed by Holmes to follow Irene early in the film.
After Holmes recreates the ritual on his own, as he describes it to Watson and Irene Adler, he holds his spotted stick to his forehead for a few seconds. This is a reference to the story "The Speckled Band", in which a speckled snake wraps itself around the villain's head and bites him.
Rachel McAdams was in Doctor Strange (2016) with Benedict Cumberbatch. Both Benedict and Robert Downey Jr. play Sherlock Holmes characters, and are in the Marvel Universe, Doctor Strange (2016) and Iron Man (2008).
The three murders of the men, and the attempted murder of Parliament, coincide with the four Greek elements. The first was a burial crime scene (Earth), second was drowning (water), third was immolation (fire), and fourth was poison gas (air).
Guy Ritchie has stated in interviews that he is a fan (and a practitioner) of Brazilian Jiu-jitsu, made popular in mixed martial arts. Towards the end of the movie, Holmes and Watson fight Dredger, and finally manage to subdue him with an arm-bar and a modified rear naked choke, both popular Brazilian Jiu-jitsu submissions.
In the newspaper with the headline "London In Terror: Blackwood Lives and the Devil Walks With Him", handed by Lestrade to Holmes when Holmes is bailed out, the names of the five women who were Blackwood's first five victims are revealed. They are: Susan Willis, Margaret Coile, June Gray, Mary Wilson, and Sarah Moss. The woman, on whose murder Holmes intervened, is revealed to be named as Beatrice Church.
All events take place in the year 1891. After Holmes and Watson are released from custody following the events in the shipyard, Inspector Lestrade hands a newspaper ("The National Police Gazette") over to Holmes. The title on the newspaper is "London In Terror", and the date is Friday, November 19, 1891, but oddly the first newspaper, foretelling Blackwoods hanging, given to Holmes by Watson in his apartment is dated Novemeber 13, 1890. This sets the story during the period when, in the original Sir Arthur Conan Doyle stories, Holmes was believed dead. According to "The Final Problem", Holmes and Professor Moriarty apparently plunged to their deaths over the Reichenbach Falls in May 1891. Holmes did not reveal he had survived until the spring of 1894, as described in "The Adventure of the Empty House".
In the last scene, when Holmes is explaining how Blackwood eluded death with a hanging rig, there is a Mook Yan Jong dummy (instrument for practicing wing chun) visible in the background as a further nod to Robert Downey Jr.'s choice of martial art.