Tom Cruise wanted to do another spy thriller, but he thought Salt (2010) and The Tourist (2010) were too similar to his Mission: Impossible (1996) franchise. He ultimately chose this project once it became an espionage thriller with comic elements.
The film's production partners, New Regency and Dune Entertainment, offset financing for the film by paying Tom Cruise a lower advance fee than he normally received. According to the Los Angeles Times, Cruise received $11 million for Knight and Day (2010), not the $20+ million he usually receives.
At one point while describing the battery, June(Cameron Diaz) jokingly comments 'The ring which never runs out of juice'- which is most likely a reference to the Green Lantern of the DC Comics. Coincidentally, the main antagonist (Peter Sarsgaard) has also worked as the main antagonist in the movie 'Green Lantern' released 1 year later(2011).