The film was acted out as an improvisation and was shot entirely in chronological order. The actors were not even given call-sheets in order to increase the film's sense of unknowing and the improvisational style. Director of photography, Dewald Aukema explains that "The film is very versatile, as there is no strict script, it is interesting that anything could happen in a particular take, and it could be used effectively in production, it's really up to the actors how they develop the scene and their characters."
According to Robert Webb in an interview with Graham Norton, they (himself and Olivia Colman) were told that they would be pixelated in the nude scenes (they play a naturist couples), and the first time they found out they weren't was at a screening on the film. Webb recalled a moment when Jessica Hynes turned round and said "Oh-ho-ho" at him. Both Webb and Colman have since spoken negatively about their experiences of the film and dissatisfaction with the end product.
The trivia item below may give away important plot points.
One of the deleted scenes was an alternate ending, where the house that Matt and Sam receive as their prize is a log-cabin in North-western Canada, and that Sam's mother was soon to join them. In consideration that the scenes were improvised, you can see Martin Freeman's look of surprise at the end of the scene.