Tonight’s episode of Sherlock saw what might be the series’ biggest twist to date, with the revelation that not only did Sherlock and Mycroft had a secret sister called Euros (or Eurus, played by relative unknown Siân Brooke), but that she’d also been stalking them in a variety of disguises as the woman John Watson cheated on his wife with (by text), a therapist and the daughter of a serial killer Sherlock was investigating.

And with the episode now ended, the question on many viewers’ lips will be how on Earth the series creators managed to keep such a big plot point a secret from fans – so it’s lucky that co-creators Steven Moffat and Mark Gatiss, producer Sue Vertue and director Nick Hurran were happy to reveal their methods at a special Bafta screening earlier today. 

“I think some of [the biggest difficulties] were trying to keep Siân under wraps,” Vertue recalled. “Where she was seen in public, where she wasn’t seen in public, and what she was as.”

“What she was as”, of course, refers to the multiple characters Brooke plays throughout last week and tonight’s episode, which Moffat explained was a tricky part of the final reveal.

“If it hadn’t worked, there wasn’t an awful lot we could do about it,” he said. “We were careful in the shots, in what you saw and all of that.

“She doesn’t actually look all that different, but it’s always to try and keep you looking somewhere else. That’s the trick isn’t it? 

“In terms of the frame, the shots…it’s your first look at Toby Jones as [new villain] Culverton Smith, so your eyes drift to him…it’s your first look at druggy Sherlock Holmes, your eyes drift to him…you’re constantly looking.

“When you go back and look at that – talk about plain sight, right there! That’s two different women, and there’s one woman playing them.”

Siân Brooke as "E" in last week's Sherlock episode

Director Hurran added: “We disguised her by… either you shoot from above, or in any of the transitions you delay how quickly you put the two together, apart from when you want to then tell that story.” 

And of course, the disguises Euros wears in her different personas was crucial to the deception.

“Actually it’s strange, blank glasses help, they sort of wipe out that bit,” Gatiss said of one of Brooke’s costumes. “Quite often you only see sort of white rectangles.”

“We only wanted to use what she could do herself,” Vertue continued, “so we didn’t want to put kinda weird prosthetics [on her].”

“I think you draw the eye with disguise,” Gatiss agreed. “It goes right back to the first series – ‘the element of disguise is learning how to hide in plain sight’ is what Sherlock says, and that’s exactly what it’s all about.

“If you have a heavy disguise you become suspicious, I think you sort of see that. Whereas is someone can just glide past, I think you’ve got them.”

“And it’s the voice as well, isn’t it?” said Vertue. “She does such a good job of speaking in that other way that all your senses are slightly fooled.”

Moffat, Vertue and Gatiss with series stars Benedict Cumberbatch, Martin Freeman and Amanda Abbington

And in the end it seems like the team pulled it off – even if they did admit they expected one or two fans to notice the twist before the end of the episode.

“One point we were genuinely worried about is the gap between episodes one and two or something getting out,” Gatiss said, “because if you start to think about it during the episode it doesn’t matter.

“Because if you think ‘Oh, isn’t she…’ it doesn’t necessarily lead to the actual conclusion, it just means something odd’s going on.”

“The trick with the twist is, it still has to work even if you get it,” Moffat agreed, likening it to his own experience watching classic movie The Sixth Sense, where he guessed the film’s famous twist ahead of the end.

“I got it quite early on, but it didn’t affect me,” the writer recalled. “I thought ‘Wow, that’s amazing!’ And I reacted with the same gasp of astonishment when what I’d known for an hour was revealed later. It still works!" 

He concluded: “A twist works if it’s a good one, whether or not it’s a genuine surprise.”

Sherlock continues on BBC1 next Sunday (15th January) at 9.00pm