The actress starring as Sherlock Holmes' surprise sister in the hit BBC drama has spoken of her relief at being able to tell friends about the role.
In a thrilling plot twist in last week's episode, Sian Brooke was revealed as Eurus - the secret sibling of Benedict Cumberbatch's Sherlock.
Speaking at a screening of the series finale, set to air on Sunday, alongside creators of the show Mark Gatiss and Steven Moffat, the actress said she had been forced to mislead friends about the character.
Brooke said: "It was amazing. I kept coming into these meetings, these secret meetings."
She added: "When they told me the news I got the part, I wasn't able to tell anybody.
"They said you couldn't tell anybody, and by that means you can't tell anybody.
"I had to tell my husband. And my mum and dad because they had to look after my kids when I was going to Cardiff."
Brooke revealed she had covered up trips to the set by telling friends she was filming a new crime drama for the BBC.
Asked if she was enjoying the secret being in the open, she said: "I am so fed up of telling lies. I'm so relieved it's out."
Last Sunday's penultimate show saw Brooke appear as three characters before it was revealed at the end of the episode they were all the same person.
The newest star of the show told fans how she had only discovered who she would actually be playing after being handed the part.
"I thought I'll have a little part in Sherlock, it will be lovely, then when they actually revealed what it was I was like what, what, what."
She added: "Joining something like this it's so exciting, it's just the most unbelievable show, the highest quality. So much care and love.
"It's just a dream come true but also so nerve-racking because the standard you have set is so high so you just want to make sure that you can reach up to where they are."
Meanwhile, Moffat promised fans a "James Bondy and actiony" episode after critics hit out at the series opener for being like a 007 film.
The show's fourth series has already seen Amanda Abbington's Mary - John Watson's wife - killed off in a thrilling build-up to episode three.
:: The finale of the BBC1 series airs at 9pm on Sunday.