As culled from various interviews, the DVD commentaries and assorted miscellany. I tried not to go with the more obvious bits of trivia (like Benedict dyes his hair) but if you’re a hardcore fan, you’ll likely still know all of these anyway:

Martin Freeman plays army doctor John Watson, formerly with the Royal Army Medical Corps. In real life, Martin’s grandfather was a medic with the 150th Field Ambulance, RAMC, during World War II. Leonard Freeman was killed in a Luftwaffe attack on May 24, 1940, during the Battle of Dunkirk.

Benedict Cumberbatch was the only actor asked to audition for the role of Sherlock, after Steven Moffat and Sue Vertue saw him in the film Atonement and Mark Gatiss, who’d worked with him on the film Starter for 10, vouched for him. Benedict’s audition was filmed in Beryl Vertue’s London flat, where she served tea and biscuits “like Mrs. Hudson.”

(The Great Game) In the scene where Sherlock examines Carl Powers’ shoe in the lab, his solo shots were done on a separate day than the shots with John. They had to shoot part of the scene without Martin Freeman after he was injured from slipping on the icy steps of a production trailer.

The pilot and A Study in Pink were filmed one year apart (in 2009 and 2010, respectively).

The dog in Hounds of Baskerville was named Whiskey after Mark Gatiss’ own childhood dog. Kirstie’s pet was an homage to Mark’s husband Ian’s childhood pet, a rabbit named Bluebell.

Steven Moffat rarely visits the set, stating that when he does, that usually means there’s a crisis going on. Parenting and Doctor Who duties keep him away. An exception is when he visited while they filmed the “flight of the dead” scene in A Scandal in Belgravia to visit his wife, Sue Vertue, as they hadn’t seen each other for several days. Production rigged up a special two person seat on the plane for them to share, but Steven promptly fell asleep.

Other people - aside from Steven Moffat - who fell asleep during the long filming of the Flight of the Dead scene include most of the extras and most of the crew, who were behind the curtain in business class. One of the grips had a vocal nightmare in which he was yelling at the focus puller.

(The Great Game) During the scenes with the elderly blind woman, the actress didn’t have her lines in advance. Instead, Mark Gatiss crouched at the foot of the bed and fed her the lines as Moriarty, to make her performance more authentic.

(The Blind Banker) The epic pen toss was done on the first take, but had to be reshot due to a problem with the camera. It took about three more tries to do it again successfully. The trick is that Benedict is looking in the mirror to know when to catch the pen.