What prompted Vivien Leigh’s dark journey into madness?

‘Vivien was barking mad from the word go,’ Laurence Olivier reflected in later life, and Lyndsy Spence’s biography would fully concur with the summing-up. At best, the actress was ‘suspended in a dream world’, unable to separate herself from the classic characters she played – Scarlett O’Hara, with her dark hair and flashing eyes, or Blanche DuBois (‘she is a tragic figure and I understand her’).