In a mind-blowing twist at the end of Sherlock series four episode two, we discovered that the detective had a secret sister, the wayward Eurus, who revealed herself to be the master of disguise behind three apparently separate characters – the Scottish girl John had flirted with on a bus in episode one, his Germanic therapist and ‘Faith’, a stand-in for the daughter of villain Culverton Smith, who Sherlock had spent an evening with while high on drugs.

It’s a development almost no-one seems to have seen coming – and for that, huge credit has to go to actress Sian Brooke who, with minimal make-up, managed to transform herself into three very different women without ever allowing viewers to guess that they were the same person.

Brooke knows exactly how the audience felt after the big reveal, though – because during the casting process she actually auditioned for each of Eurus’s characters separately, never having a clue that they would turn out to be one and the same role...

“When I found out I’d got the part I felt exceptionally lucky to be part of something, to join something, that I’d loved for such a long time,” said Brooke, speaking after a screening of series finale The Final Problem. “And then when I discovered exactly what it was I couldn’t quite believe it. I didn't find out until I got the part that I was going to be his sister. It was a revelation!”

“We got Sian, and a lot of other people to audition… as if there were several parts, like Faith and the girl on the bus,” revealed Sherlock co-creator Mark Gatiss. “And then it was only at the eleventh hour that we said ‘Actually they’re all the same person’.”

“It was amazing,” added Brooke. “I kept coming in for these meetings, these secret meetings, and thinking ‘Why am I [being asked to audition for all these parts]? Oh, there’s just a couple of [small] parts and a big part', that’s what I thought. I thought ‘Well, it’s a part in Sherlock, lovely.’ And then when they actually revealed what it was I was like ‘What? What?! What?!’"

It was a clever way for Gatiss and co-creator Steven Moffat to see how convincingly Brooke would be able to perform each role within the episode, and also to help guard the secret of Eurus’s identity from her until they knew she was the one they wanted.

Brooke said landing the part had been “a dream come true” but also “nerve-wracking”, thanks to the quality of the show she was working on. 

“Joining something like this, it’s so exciting," she said. "It’s just the most unbelievable show – the highest quality, so much care and love – so to join it is just a dream come true."

Of course, we haven't seen the last of Eurus, she will return for The Final Problem. And it's safe to say it's another chance for Brooke to show off her versatility as an actor.

Sherlock: The Final Problem is on Sunday 15th January at 9pm on BBC1

Sherlock Series 4 DVD is released on 23rd January