The film is directed by Bill Condon, who worked with McKellen on Gods and Monsters and Mr Holmes.
The Good Liar, a thriller full of twists and turns, tells the story of ageing conman Roy, who preys on a wealthy widow, Betty, after meeting her online.
It was reassuring, but quite a shock just how living and breathing on the screen Roy and Betty were.”
He was taken aback when publishers snapped up his first novel and turned it into a bestseller in 2016.