When Anna (Stefania Rocco) is attacked in her home, (suddenly) a pair of dismembered fingers are shown; this is left in / over from shots not included the film, first inferred when John (Liam Cunningham) inspects / lifts the first victim corpse's arm in the mortuary, and then to the water recovered victim, which in a short location 'making of' documentary, is shown as also had missing fingers.
The idea behind 'The Card Player' came in 1996 just after the release of 'The Stendhal Syndrome' and was intended to be a sequel to 'The Stendhal Syndrome' titled 'In The Dark' with Asia Argento returning to portray the character of Anna Manni, but director Dario Argento then decided to make 'Phantom of the Opera' instead and the idea was abandoned.
The role of the cop was intended for Mathieu Kassovitz but he withdrew when he was offered the chance to direct Gothika (2003). It was eventually given to Liam Cunningham and the name changed from John Russo to John Brennan.
Dario Argento stated in an interview with Alan Jones that Stefania Rocca was an actress he could actually talk to about film theory. Which lead to Dario being more flexible working with Stefania compared to the other actresses he has previously worked with.
Stefania Rocca spent a lot of time with four Roman policewomen to witness them in action throughout their daily routines.