In the public mind, Dame Diana Rigg will forever be linked with performances which were almost a sideshow in her long career. Her roles as Mrs Peel in The Avengers and as Bond girl Tracy di Vicenzo in On Her Majesty’s Secret Service attracted huge attention, but it’s in the theatre that she's won critical acclaim and a host of awards. Now she has an iconic role in the hit series Game of Thrones - what's the secret to a great acting performance?Picture: Diana Rigg, Credit: Tim P. Whitby/Getty Images