Who played Sherlock Holmes in 'Sherlock'?

Who played Nucky Thompson in 'Boardwalk Empire'? In which TV series is Nucky Thompson a main character? Who played Violet Crawley in 'Downton Abbey'? In which TV series is Violet Crawley a main character? Who played Dr. John Watson in 'Sherlock'? In which TV series is Dr. John Watson a main character? In which TV series is Tyrion Lannister a main character? Who played Cersei Lannister in 'Game of Thrones'?