During the "memory palace" scene in "The Hounds of Baskerville," one of the words displayed is Ingolstadt, a German city that features prominently in Frankenstein. In Danny Boyle's stage adaptation of the novel, Benedict Cumberbatch alternated the roles of Victor and the Creature with Jonny Lee Miller, who coincidentally is Sherlock Holmes in Elementary.
In "The Reichenbach Fall", Sherlock tells Lestrade that "You can't kill an idea", something he should know, considering his detective work in V for Vendetta.
The webisode "Many Happy Returns" also features a case of Creator Allusion. Lestrade gives John an uncut video of a birthday greeting Sherlock made for him. Watching the video, with Sherlock futzing about with Lestrade, John says "I can tell you what you can stop doing. You can stop being dead." Sherlock, as though he is talking to John through the video, replies "Okay."
A commenter on John's blog has the screenname "Sauron1976". Benedict Cumberbatch plays the Necromancer (who becomes Sauron) in The Hobbit. Benedict was also born in 1976.
In one of the possible ways Sherlock faked his death, Mycroft, played by Mark Gatiss is called upon to help fake Sherlock's death via something called "Lazarus". He also starred in an episode of Doctor Who called The Lazarus Experiment, in which he played the titular Professor Richard Lazarus.
During The Empty Hearse, Sherlock faked a French accent - which Benedict Cumberbatch also did in the Cabin Pressure episode Qikitarjuaq.
In "The Empty Hearse", John puts a surgical glove on middle finger first; flipping the bird is a frequent habit of Martin Freeman's.
Sherlock identifies a person, albeit falsely, though scent.
Then there's Sherlock and Mycroft's conversation about dragons.
Dueling Shows: With Elementary, its CBS cousin. Sherlock has a two-season head start, but Elementary's pushing out more episodes (So far, time wise it's ahead) and got a Superbowl episode in March 2013.
Dyeing for Your Art: Both Benedict Cumberbatch and Mark Gatiss dye their hair black for their roles as Sherlock and Mycroft. They're both naturally ginger.
In preparation for the first season, Cumberbatch lost some weight to reflect Sherlock's tendency to not eat while working. He's even thinner in the pilot. In the second season, he gained the weight back - after all, Sherlock now shares an apartment with a physician.
Edited for Syndication: "The Empty Hearse" for the American broadcast on PBS. In the series of intercut edits between Sherlock and John, Sherlock's "Fuuuu" (that feeds into John's "cough" to create "fuck off") is cut down by a few hundredths of a second so it's not as blatant. You would have to watch both versions to notice. Similarly John's swearing ("You - you cock!") is edited so that he just says "You!" twice. However, his calling Sherlock a "complete dickhead" is left intact in "The Sign of Three".
Enforced Method Acting: Lara Pulver caused quite a stir when she admitted that during the scene where Irene Adler was naked in front of Sherlock, she really wasn't wearing anything. One wonders how much of Martin and Benedict's reactions were genuine.
John's comment on Sherlock "ripping his clothes off in a darkened swimming pool". For the unassuming public, a fun nod to the Ho Yay.
John directly addresses the homoeroticism in "A Scandal in Belgravia"—
John: Who the hell knows about Sherlock Holmes, but for the record — if anyone out therestill cares — I'm not actually gay.
There is no way, none at all, that the jumper John is wearing during the Christmas interlude wasn't inspired by the fandom's appreciation of John Watson's cuddly jumpers.
Sherlock mentions "the depth and complexity of John's jumpers" in The Sign of Three while rambling on distractedly during his best man speech at John's wedding.
When Sherlock and Irene first meet, one of her first comments is about his razor-sharp cheekbones, saying she could cut herself slapping that face. Those cheekbones have their own little fandom.
John's "Asperger's?" remark in "Hounds" seems to be this.
While cuffed together, Sherlock and John have to run while holding hands. John quips, "Now people will really talk."
Pretty much everything in 'The Empty Hearse', especially Anderson's club devoted to theories about how Sherlock may have faked his death. We also get nods to just about everything the fandom has been known to ship.
Fan Nickname: "Mocklock" has been used in a few comment threads for those that believe that the ambassador's children were kidnapped by someone disguised as Sherlock Holmes, which caused the little girl to freak out when she saw the real Sherlock. Also used to refer to the theory that the dead Sherlock at the end of the episode was a body double, possibly the same body that kidnapped the children.
Since the two producers work so closely together on this, their names have slowly condensed from "Steven Moffat and Mark Gatiss" to "Moffat and Gatiss" to "Moffat-Gatiss" to finally colliding into "Mofftiss."
When speaking only of Mark Gatiss, fans usually switch his surname to God-tiss.
In the Chinese fandom, Sherlock and John are nicknamed "Curly Fu" and "Peanut". "Curly" refers to Sherlock's hair and "Fu" is short for Fuermosi (福尔摩斯), the Chinese rendering of "Holmes". "Peanut" refers to the Chinese rendering of "Watson", Huasheng (华生), being a homophone of the word for peanut, huasheng (花生).
Emma's actress was, in real life, Benedict Cumberbatch's long-time romantic partner. Puts an interesting spin on Sherlock interrogating Amanda (the murder victim's mistress in episode 2) about her relationship.
Major Sholto is Alistair Petrie, an actor who also featured on Jonathan Creek. This is notable since in that series his character also faked his own death by leaping from a high building. Bonus points for Jonathan Creek creator David Renwick accurately guessing how Sherlock did it by comparing it to his own episode, in which both characters more-or-less use the same technique in surviving the fall Sherlock landed on a huge inflatable device; Petrie's character had pre-rigged an underground trampoline that was hastily covered up by a grass verge.
Also, any fans of The Goon Show already knew that thanks to their character "Count Jim Knees [Amusing Noise].... Moriarty...
The way Sherlock faked his death incorporated his homeless network, the man on the bike, Mocklock, the truck, a giant cushion, and a ball under the armpit. All were components of fan theories, and it's likely someone out there got everything right.
Real-Life Relative: The producers have joined in on the good-natured ribbing about how much they keep it in the family when making the show.
Amanda Abbington, Martin Freeman's partner, has officially been cast as Mary Morstan for series 3.
While Sherlock's parents were played by Wanda Ventham and Timothy Carlton, Benedict Cumberbatch's own parents.
Throw It In: Moriarty wasn't scripted as dancing as he breaks into the glass chamber with the royal jewels, but the music was actually playing on set and Andrew Scott decided to dance anyway.
When Sherlock and John are drunk in The Sign of Three, Sherlock says "I apologize for my...my...thing," seemingly unable to come up with the word he wanted to use to describe John (who had fallen asleep when their client was talking). Benedict Cumberbatch had forgotten his line, but they thought it was hilarious and left it in.
Troubled Production: The first series had quite a few problems going for it, which started with the fact that it was being filmed during one of the coldest winters in the UK in nearly 30 years. This led to Benedict Cumberbatch catching a very dangerous case of pneumonia, and Martin Freeman severely hurting his wrist after slipping on an icy step coming out of his trailer. The second season only faired slightly better, no severe damage, but one day of filming was halted by the London riots.
Matt Smith auditioned to play John, but was turned down for being more of a Sherlock (they also kind of hinted that he might have been Sherlock, except they were already set on giving the part to Benedict.)
When comparing the characterization of Sherlock in the unaired pilot and the canon, Sherlock comes across as far more sympathetic in the pilot. Considering the actions Sherlock is capable and willing to do in the series, this could ironically enough be a way of making him less creepy.