Kitty Riley : Katherine Parkinson - IMDB page
An ambitious journalist, Kitty is looking for a scoop, and ready to confront Sherlock for that. The first meeting turns to an humiliation, Sherlock being haughty and contemptuous. But Moriarty decides to use her as an asset, and she will play a good part in Sherlock's fall.
Ella, Psychotherapist : Tanya Moodie - IMDB page
John's therapist; he came after his return from Afghanistan, but Sherlock, as a provider of action and adrenalin, proved to be much more efficient than a therapy, and John stopped the appointments. When his best friend dies, he needs to resume it.
He doesn't understand why Lestrade is so trusting and accommodating with Sherlock. Donovan and Anderson's suspicions seems much more justified. After all, if Sherlock is not a psychopath, he is, for sure, a "weirdo". Something the Chief Supertintendant should not have said before John, for the sake of his nose...
Judge : Malcolm Rennie - IMDB page
It' s probably because he is a seasoned judge that he got the trial of Moriarty to run. Not a gift at all... Intelligent and blessed with a strong character, the judge has no other option than accept the "non-guilty" verdict. Moriarty made sure the jury would ask for it.
Moriarty's defence barrister : Ian Hallard - IMDB page
This lawyer is as young as the judge is seasoned, but Moriarty needs no one to defend himself. He will call for no witness, will say nothing. Blackmailing members of the jury is much more efficient.
A small role, but Mark Gatiss seized this opportunity to stage a scene with his real-life partner.
