Charlie Cooper gained a new appreciation for the "mathematical" precision of a great whodunnit story while appearing in the 1950s-set thriller See How They Run.The This Country actor plays theatre usher Dennis in the new murder-mystery, which is set amid the glamour of a West End production of Agatha Christie's classic thriller The Mousetrap.The discovery of a dead body behind the scenes causes the roulette wheel of potential suspects to spin, while cops played by Sam Rockwell and Saoirse Ronan aim to track down the killer."The challenge is with the writers, I think. It's so formulaic, isn't it — a murder-mystery?" Cooper told Yahoo.See How They Run hits UK cinemas on 9 September.