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!

3.2 The sign of the three

Sherlock is terrified: John asked him to be his best man for his marriage. Sherlock has a speech to make. A real speech in front of real people.

He will do it in his so particular way, avoiding a murder at the same time.