Which bestselling novel was adapted as a 2014 film starring Nicole Kidman?

Before I Go to Sleep is a 2014 British-American mystery psychological thriller written and directed by Rowan Joffé. A film adaptation of S. J. Watson's 2011 novel of the same name, the film stars Nicole Kidman, Mark Strong, Colin Firth, and Anne-Marie Duff.

Before I Go to Sleep is the first novel by S. J. Watson published in Spring 2011. It became both a Sunday Times and New York Times bestseller and has been translated into over 40 languages, and has become a bestseller in France, Canada, Bulgaria and the Netherlands. It reached number 7 on the US bestseller list, the highest position for a debut novel by a British author since J. K. Rowling.

On 1 May 2011, Ridley Scott bought the film rights of the novel Before I Go to Sleep written by S. J. Watson and hired Rowan Joffé to direct and write the script. In February 2012 Nicole Kidman was in talks to join the film; later in May she joined to star as Christine. On 31 October 2012 Mark Strong also joined the cast of the film. According to The Hollywood Reporter on 16 November 2012, Kidman said she wanted Colin Firth to work with her again. They had already worked together in a drama The Railway Man. On 6 February 2013, it was confirmed that Firth has joined the cast to lead with Kidman.