Sean Harris is a method actor and didn't socialize with anyone throughout the shoot.
It took seven hours a day to transform Sean Harris into Craig and three hours to remove all the make-up at the end of a day's shooting.
The Charing Cross station where a large part of the movie was set is actually a disused platform on the Jubilee Line. It was closed in 1999 but is often used by film and television makers requiring a modern Underground station location.
Kelly Scott didn't see Sean Harris in full make-up as Craig until the sequence where Craig operates on Scott's character Mandy was filmed. As a result, many of Scott's reactions of fear and confusion were genuine.