The original script was set in the United Kingdom.
The formula wDz=Cov(wi,zi) is the Price equation by George R. Price. It describes how in a reproducing population traits that increases fitness will increase with each new generation.
The sets made in Belfast to simulate New York City were built in under two weeks time.
Associate producer Brock Norman Brock, supporting actor Tom Hardy, and cinematographer Morten Søborg shortly after this film would work with Nicolas Winding Refn. Brock (as co-writer) and Hardy (in the lead titular role) in Bronson (2008) and Søborg (who is famous for his work with Refn) in Valhalla Rising (2009).