The character of Greg Lestrade is a combination of Inspectors Gregson and Lestrade. In the books, the latter character's first name is said to begin with the letter G, but is never revealed. This is why Sherlock is always forgetting Lestrade's name.
In this series, Watson was wounded in the shoulder but has psychosomatic/psychogenic pain in his leg. This is a sly reference to the original stories in which Arthur Conan Doyle was inconsistent about the location of Watson's war wound.
The role of Molly Hooper was never in the books or short stories and was only meant to be a one-off character to further indicate Sherlock's lack of social skills, particularly addressing any romantic encounters. However, Steven Moffat and other producers loved Louise Brealey's performance so much that they decided to expand her character.
Sherlock sometimes uses a memory technique that he calls a "Mind Palace." This is not an invention of the screenwriters; rather, it is a method of aiding memory that dates back to Ancient Rome called the "Method of Loci". One of history's most famous real-life practitioners was the sixteenth-century Italian Jesuit missionary Matteo Ricci, who introduced the method to China.
Through the series, several Holmes stories by Arthur Conan Doyle are alluded to, either as episode names or in passing. For instance, "A Study in Scarlet" becomes "A Study in Pink", "The Greek Interpreter" becomes "The Geek Interpreter" etc.
In Sherlock: A Scandal in Belgravia (2012), Sherlock tells Adler about "Coventry". He says "In the second world war the Allies knew that Coventry was going to get bombed because they'd broken the German code but they didn't want the Germans to know they'd broken the code so they let it happen anyway". Cumberbatch later played Alan Turing, the mathematician who broke the German enigma code, in The Imitation Game (2014).
When Martin Freeman arrived for his first audition, he just had his wallet stolen. He was in such a bad mood that the producers thought he wasn't interested in the role. Luckily, for the next audition he was fine, and got the role.
As in the Sir Arthur Conan Doyle stories, the term "deduction" is misused. Like medical diagnosticians, hunters and yes, detectives, what Holmes actually uses a form of inference called "abductive reasoning," which is neither deductive nor inductive.
At one point in the series, Watson gets fed up with hearing Holmes talking about his intellect and how emotions are unimportant to the point that he sarcastically calls Holmes "Spock". Benedict Cumberbatch appeared in Star Trek Into Darkness (2013) as Kirk and Spock's arch-nemesis, Khan.
"Jim" Moriarty's appearance is subtly the exact opposite of Sherlock's; Sherlock's hair is long and curly, while Moriarty's is clean and combed. Sherlock wears an open collar while Moriarty always wears a tie. As well, Sherlock dons his classic long overcoat with his high collar popped, while Moriarty's is a shorter, trimmer jacket with a Nehru collar. And in most cases, Sherlock is clean shaven whilst Jim wears light five o'clock shadow on his face.
In the summer of 2011 Danny Boyle created a National Theatre production of 'Frankenstein' in which Benedict Cumberbatch and Jonny Lee Miller played the creator and monster and alternately changed nightly. Both actors then went on to play another Victorian creation Sherlock Holmes, both set in the present day, allbeit opposite side of the Atlantic.
Watson's reference to Holmes as "Spock" has yet another significance. It is implied in Star Trek VI: The Undiscovered Country (1991) that Sherlock Holmes is an ancestor of Spock's, "An ancestor of mine maintained that if you eliminate the impossible, whatever remains - however improbable - must be the truth."
Although John and Sherlock aren't the wealthiest of people and Sherlock is frequently seen on different occasions denying any reward in money for his job, Watson's clothes do not reflect this. In fact his famous black jacket costs £750, shoes £175, knitted jumper over £200 and his watch a staggering £3000. Sherlock's famous coat is also a staggering £1,750.
Watson's reference to Holmes as "Spock" has another significance. The original Spock, Leonard Nimoy, has also played Sherlock Holmes. Nimoy also recorded a song called "The Ballad of Bilbo Baggins," based on The Hobbit. Martin Freeman and Benedict Cumberbatch appeared together in the film adaptations.
In this new series Dr. Watson used to be a Captain (and medical doctor) serving with the 5th Northumberland Fusiliers deployed to Afghanistan. The 5th Northumberland Fusiliers did indeed serve in Afghanistan in the Second Afghan-Anglo war (1878-1880). The first story of the old Sherlock Holmes series took place in 1881. The 5th Northumberland Fusiliers, however, was renamed into the "The Northumberland Fusiliers" in the year 1881 (no 5th left in title), which means the modern Dr. Watson service with the 5th Northumberlands is a nod to its old heritage. The former 5th Northumberlands evolved into its last formation named "Royal Northumberland Fusiliers" and was amalgamated in 1968. Watsons current Regiment would be 'The Royal Regiment Of Fusiliers', with whom have served in Afghanistan multiple times over the past 16 years.
Benedict Cumberbatch, Robert Downey Jr. and Ian McKellen have all played the character of Sherlock Holmes before. All three of them have also played roles in Marvel movies. Cumberbatch plays Dr. Stephen Strange, Downey plays Iron Man and McKellen played Magneto.
In the Season 2 premiere, Sherlock talks of the 'myth' about the Allies knowing about the Coventry bombing and still letting it happen so that they don't disclose that they have cracked the enigma code. In the movie The Imitation Game (2014) Benedict Cumberbatch plays Alan Turing and the entire scene is played out on the screen.
In Conan Doyle's original stories, Sherlock Holmes and Dr. Watson addressed each other by their last names, which was common during Victorian times. Since nowadays it is more common for people to address each other with the first name, the show has the two characters address each other as "Sherlock" and "John".
Watson's P226R is likely meant to be his army service pistol. Although the P226 is used in the British Military, it is mainly used by Special Forces and it is rare to be held by Infantry, where the Browning Hi-Power or the Glock 17 Gen 3, which is phasing out both handguns for its high magazine capacity and reliability, are more common.
In A Study In Scarlet-the original story- Scotland Yard finds "rache" on a wall written with blood, and Lestrade thinks that the victim was trying to write Rachel. Then Sherlock says that it was revenge in German. In Sherlock: A Study in Pink (2010) , the victim claws 'rache' into the floor. Anderson, the forensic expert, thinks that the victim was trying to write revenge in German, but Sherlock reckons the victim died before finishing to write 'Rachel'.
In Sherlock: A Scandal in Belgravia (2012) the scene where Sherlock is unlocking Irene Adlers phone, he types the numbers "7437" which are letters "SHER". Because of that, the letters make a sentence "I AM SHERLOCKED".