Recycled Script: The episode is an update of the first Holmes tale "A Study in Scarlet".
Throw It In: According to the episode commentary, Sherlock kissing Mrs Hudson on the cheek wasn't scripted. Benedict Cumberbatch had known Una Stubbs since he was very young, and it was his reaction on being able to work with someone who was like a mother to him.
Troubled Production: Originally this was shot as a 60-minute one-off TV movie. Its costs ballooned to an almost unheard-of one million pounds, and rumors flew that the BBC would pull the plug and leave it unaired. Instead, they asked Mark Gatiss and Steven Moffat to turn the whole thing into a series pilot. That would entail making it longer. And they wrote a longer script. But they couldn't get the same cinematographer back, and didn't think the added scenes would be consistent with what they had already filmed. So they shot the whole thing over.
They had to do this, with many scenes taking place at night, outdoors, or both, during what turned out to be the coldest winter in Britain for three decades. Martin Freeman recalls standing in ice water once for several hours waiting to get a take done. One morning he fell while getting out of his ride to the set and broke his wrist, requiring that he shoot many of his scenes for the next several weeks with his arm temporarily liberated from its plaster cast. He was luckier than Benedict Cumberbatch, though, whose persistent flu turned out to be life-threatening pneumonia, for which he was hospitalized.