Who played Violet Crawley in 'Downton Abbey'?

Who played Negan in 'The Walking Dead'? In which TV series is Negan a main character? Who played Nucky Thompson in 'Boardwalk Empire'? In which TV series is Nucky Thompson a main character? In which TV series is Violet Crawley a main character? Who played Sherlock Holmes in 'Sherlock'? Who played Dr. John Watson in 'Sherlock'? In which TV series is Dr. John Watson a main character?