Say the word 'Wicked' and most of us think of the Broadway smash hit. The man behind the untold story of the Witches of Oz' is composer and lyricist Stephen Schwartz. It can be seen as a commentary on the nature of good and evil. Schwartz speaks with Zoë George about spotting talent, what he thinks of other people’s interpretations of his work, and the secret to writing a successful song.