Shel's phone-call (in Swedish) was entirely improvised by MyAnna Buring. The filmmakers had no idea what she said until much later.
The rainbow featured prominently in the film is a real one. There was a rainbow written into another scene, but it was changed when the crew saw one during the shooting of the scene.
When Jay and Gal are running in the tunnels, the shark equivalent of whale song is played on the soundtrack.
The librarian telling Jay "thank you" was cowriter Amy Jump's idea. "It's scarier if someone asks for it," she adds.
Ben Wheatley says a normal horror movie would feature monsters jumping out, but he's seen a great response to the more relatable horror of hearing your parents yelling at each other.