Sherlock creator Steven Moffat has said that actress Louise Brealey, who plays Molly Hooper, has made them break their own rules.
In an interview with Digital Spy, he explained why he decided to break his first rule of not introducing new regular characters.
He said: "She's really interesting, Molly, because she was an absolute one-scene character for the pilot but Lou Brealy was just so fantastic.
"We went against our first decision which was 'We will not add a regular that's not from Doyle'. The first thing we did was add a regular character that's not from Doyle!"
He added: "I think she's fascinating because over time, certainly by the time you get to the second series, she wins every encounter with Sherlock [played by Benedict Cumberbatch].
"All the time, always. And by being honest and truthful with him. He's so on the back foot now with Molly, I think it's hilarious."
He added: "In a way that John can never put Sherlock on the back foot, Molly really, really does. She sort of wins every single conversation."
Co-creator of the popular adaptation of Arthur Conan Doyle's novels, Mark Gatiss added: "It's a fascinating thing because it's a new idea. We were doing the pilot and I said, 'What if she has a boyfriend and Sherlock says he's gay?'.
"That rolled on to the whole thing with Jim and it massively expanded. It's so much to do with Lou's interpretation making it so heartwarming."
Moffat agreed: "It's extraordinary. What you don't get a lot of in the original stories is, 'What do women think of him?'.
"They don't talk much in the original stories. It doesn't happen much. There's Irene Adler who he barely meets in the original. There's Mrs Hudson who just brings tea in.
"It was fun to have that perspective on him, what would a modern young woman make of this vulnerable monster."
Sherlock returns to BBC One on New Year's Day at 9pm