As the Oscar-winning Stasi surveillance film hits the stage, Knightley and co-star Luke Thompson talk about the paranoia at its heart, the cost of theatre tickets nowadays – and their fury at ringtones in the stalls

Unpacking assorted tubs of carrot sticks and salad, Keira Knightley winces apologetically. “I just realised I’ve got very crunchy food,” she says. How apropos that she should consider the impact that her lunch might have on my audio recording when we are here to discuss The Lives of Others, a new play in which the characters are divided between the spies and the spied-upon.

Based on the Oscar-winning 2006 film and set in East Berlin in 1984, the play stars Knightley as Christa-Maria, a theatre actor in a relationship with a playwright, Georg, played by Luke Thompson. Earwigging on them is Wiesler (Stephen Dillane), the Stasi officer whose principles are gradually challenged and weakened by his round-the-clock exposure to this cultured couple.