Henry Cavill dropped out of the title role only a week before filming was due to begin due to "creative differences". He was replaced by Dominic Cooper.
Duncan Falconer (the author of the Stratton series) is a former member of the British S.B.S., Special Boat Services, a Royal Navy special forces unit equivalent to the S.A.S. Falconer was active during the Falklands War and worked with Intelligence in Northern Ireland.
The memorial plaque to William Donald, beside which Stratton and Sumner pause while walking along the Thames, refers to a real event. The plaque's actual location is not on that riverside wall, but in Postman's Park in the City of London, a few miles northeast of the location depicted in the movie.
Ross (Derek Jacobi) mentions in a conversation he is about to be knighted. Jacobi has, in fact, been knighted in two different countries: Great Britain and Denmark; a distinction given only one other actor, Laurence Olivier (who was ultimately given a peerage by Queen Elizabeth).