David Nicholls and James Hogg Adaptation and Ghost Writing

How does a writer take their work from a novel to the screen, or step into someone else's shoes to write their story in their own words? The novelist David Nicholls is a scriptwriter and screenwriter, and has adapted Shakespeare for screen and his own novels, including Us and One Day, for film the latter out this year. James Hogg is a ghost writer and biographer who has ghostwritten or collaborated on autobiographies for leading actors including Timothy West, Richard Briers and Brian Blessed, and racing driver Damon Hill.