Joshua David Hall has said that his three main inspirations for this film were Irreversible (2002), Zero Day (2002) and Dead Man's Shoes (2004).
The opening quote "revenge is not a human right, but it is a human reaction" is a paraphrasing of a comment made by Vincent Cassel when he was doing promotion for Irreversible (2002).
The story was influenced by a short film Joshua David Hall made the year before, called 'Separation', which was about a teenager (played by Ben Arnold) who kidnaps a bully (played by Eric Bay-Andersen) and takes him to the woods to kill him. Ironically, in this film the roles are reversed - here, Eric Bay-Andersen plays the kidnapper and Ben Arnold plays the victim. The short can be found on YouTube.
The film-makers mainly worked from an outline. The only scenes that were fully scripted were the scene in the bar where Dan and Matt first discuss their plan and the scenes in Brighton. Other scenes, like the one where they torture Shane, were completely improvised.
In the scene where Dan and Matt pull into the garage area, Michael Roberts thought the scene was going to end with Eric Bay-Andersen shutting the car trunk (he was lying underneath the car rather than in the trunk). When Eric turned the engine on and reversed out, Michael got a face full of smoke from the exhaust pipe and had to roll out of the way quickly to avoid being run over.