Actress Sian Brooke has said she was “blown away” at landing the part of Sherlock’s dangerously intelligent sister, Eurus Holmes, in the BBC drama.

Describing the part in the show’s latest series as a “dream role”, she said she had only expected to make a short cameo appearance.

Commenting on the role on BBC Breakfast on Monday, hours after the fourth season finale was broadcast, she said: “I have been gripped by Sherlock since it started.

“When they said I had got the part, I was blown away – it was the dream role for many reasons.”

Speaking about taking on the role of the psychopathic genius, who has spent her entire adult life in a high-security institution following her murderous childhood, Sian said: “It’s a joy as an actor to be able to play a part like that, which is, hopefully, so far removed from myself.

“But it’s also quite a challenge because I was playing something that was void of any emotion.”

Explaining that she originally thought she would be scripted in for a “nice little cameo role”, she said she noticed things were “awry” as she was given more lines.

She was also “sworn to secrecy” during filming for the hotly anticipated three-part series and had to make up excuses about why she was travelling to Cardiff for weeks at a time.

“I had to sign my life away from the start – I couldn’t tell anybody that I was doing it,” she said.

“People asked where I was going so I just said I was doing a crime drama for the BBC.

“When they asked about it or who was starring in it, I just told them it was nobody they would know.”

Asked if she had been contracted for a further Sherlock series, she said: “I would love there to be another series, but it’s up to the powers that be to make that happen.”