See How They Run's Charlie Cooper praises 'meticulous, mathematical' script (exclusive)

Watch: Charlie Cooper discusses the "meticulous" writing behind a whodunnit

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.

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"The challenge is with the writers, I think. It's so formulaic, isn't it — a murder-mystery?" Cooper told Yahoo when asked how hard it is for actors to be the right amount of suspicious to keep audiences guessing.

"It's so mathematical. If you linger on a character for too long in one scene, it's sort of a red herring or whatever."

Cooper said that See How They Run screenwriter Mark Chappell — a veteran of the British comedy scene — did a great job of making this particular mystery work.

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He added: "The way it was constructed and written was really meticulous, so as an actor you don't actually have to do much. You just do what's on the page and, luckily, it's all there.

"I'm really bad at knowing [the killer in whodunnits]. My girlfriend gets it straight away. With this, just go with the ride, enjoy it and don't try to guess too soon."

The whodunnit genre is in the midst of a renaissance on the big screen, whether it's Kenneth Branagh's lavish Agatha Christie adaptations or Rian Johnson's Knives Out franchise.

"I personally think that, on a global scale, they come and go. But in the UK, the whodunnit is a constant thread running through," said See How They Run director Tom George.

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He added: "Barely a year goes by that there's not a big money adaptation of a murder-mystery in some way.

"But I think what's exciting for me, with this film, was how do you in that space do something new and slightly unexpected, while also paying homage to the whodunnits of the past?"

By setting the murder-mystery at a production of The Mousetrap, the story exists as a whodunnit within a whodunnit, packed with nods and references to the genre's past while avoiding spoiling Christie's classic.

Cooper said: "With Agatha Christie, you know more [about her work] than you think you do. It's part of our history, I suppose. [The Mousetrap] has done 29 and a half thousand shows. That's mad.

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"There isn't anyone who doesn't love a good murder-mystery. It's definitely British — something about that morbid curiosity that we have, like that Jack the Ripper stuff. It all comes from him. He started this, so it's his fault."

See How They Run hits UK cinemas on 9 September.

Watch: Trailer for whodunnit crime thriller See How They Run