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.
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.
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."
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.
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.
Matt Smith auditioned to play John, but was turned down for being more of a Sherlock.
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.