In Moody's room (where he received his birthday present) two paintings are visible. The one on the left is Giuseppe Arcimboldo's 'The Summer' (1563), the one on the right is Arcimboldo's 'Spring' (same year). Arcimboldo painted the portraits in what is called "double images paintings" in which a main image is created with other items, in this case, the figure of a face is built with pieces of plants, flowers, etc. It could represent double nature in both main characters: assassins and regular law-abiding citizens.
Despite Maggie Q being the protagonist, Michael Keaton is the first to appear in the credits.
In Moody's basement ops room, and in the back (computer ops) room of the laundry there are the same maps on the wall. The maps are of the Tigris river at Al Kut in Iraq.
Both Maggie Q and Samuel L. Jackson have starred in the Die Hard series. Maggie starred as cyberterrorist Mai in Live Free or Die Hard (2007) while Jackson starred as Zeus, McClane's civilian partner in Die Hard with a Vengeance (1995).