Casting 3.3 His last vow

Lady Elisabeth Smallwood : Lindsay Duncan - IMDB page

As an MP, Lady Smallwood leads a commission of inquiry about a possible influence of Charles Augustus Magnussen, a powerful media mogul, and the British government. Magnussen has visited 10 Downing Street seven times over the year, but refuses to give any explanation. Magnussen and Lady Smallwood are both cold-blooded, but Magnussen is unscrupulous and blackmails his opponent. Lady Smallwood will need the help of the only person who doesn't fear Magnussen: Sherlock.

Charles Augustus Magnussen : Lars Mikkelsen - IMDB page

Magnussen is as powerful in GB as Mycroft himself -aka he is the most powerful man, ex-aequo. Why did he choose to operate in/from Great-Britain? Because British "are a people of ruminants". Proud and contemptuous, Magnussen is a freaky villain, however, he has no blood on his hands (not directly!). He threats, hurts, gives, leads to suicide, but doesn't kill himself. But he will provoke Sherlock's hate though, after having succeeded in trapping the genius.

Casting 3.2 The sign of the three

Major James Sholto : Alistair Petrie - IMDB page

An ancient officer highly estimated by Watson, Major Sholto gained a lot of enemies. After a failed mission, several young soldiers died, and their families take Sholto for responsible. That's why Sholto lives now retired from active duty and life. But he makes an exception for John's wedding, and faces an unexpected hostility: Sherlock's jealousy!

Casting 3.1 - The empty hearse

Mary Morstan : Amanda Abbington - IMDB page

John's fiancée. Despite the special mood of the reunion between both friends, she likes Sherlock straightaway. She doesn't fear his look upon her. Neutral when John refuses, at first, to forgive, she will then support their reconciliation.

Casting 2.3 - Falls of the Reichenbach

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.

Chief Superintendant : Tony Pitts - IMDB page

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.

Casting 2.2 - the hounds of Baskerville

Henry Knight : Russel Tovey - IMDB page

Orphan at ten, witness of the murder of his father, Henry succeeded in building his life. He is still seeing a therapist, though. But as memories are coming back, new events make him feels weak and insane.

Casting 2.1 A scandal in Belgravia

First of all, "The woman" :

Casting 1.3 - The great game

This episode doesn't lack of excellent characters ! I made a subjective choice with the list below :

Casting 1.1 A study in pink

In this introductive episode, all main characters are present (aside Mary Morstan and Bill Wiggins, season 3). Below are the characters appearing only (or mainly) in this episode.

Casting 1.2 - the blind banker