In the film, Kelly tells her superior, Ed, that she turned fifty last week. Actor Caroline Spence actually did turn fifty a week after filming the scene.
As a non-professional actor, Caroline Spence decided to adopt an American accent to help her acting performance: "otherwise I'd just sound like me."
Whilst shooting a chase scene in Andalucía on the edge of a nature reserve with actor Eddie Gallego, the team had to contend with the largest plague of mosquitoes ever recorded in the region.
It was the original intention to shoot the film in script order over only three weeks. This didn't happen, however, as the filmmakers had to return to the UK unexpectedly before other key Spanish scenes were shot. They picked up filming two months later and finally wrapped in Almería, Spain on 5 July 2017.
Inspiration for the radio debate at the beginning of the film came from Duel, the directorial feature debut of Steven Spielberg.