According to the DVD commentary with Josh Hartnett and Lucy Liu, Lindsey's reaction when she walks in on Slevin with his towel open is real. The scene was scripted but Josh Hartnett actually flashed Lucy Liu without telling her prior.
Josh Hartnett (Slevin) lived with writer Jason Smilovic and his girlfriend in New York while the script was being written. Smilovic said that he thought of having Slevin wear a towel a lot of the time because he saw Hartnett in one so often. More so it added a vulnerable quality to Slevin.
On promotional material for the film, Ben Kingsley's credit included his knighthood. At first, the actor was singled out for some criticism as such things are usually omitted from professional credits. It transpired that this was a mistake by a studio executive, who was unfamiliar with the British honours system.
In the film it is thought that Mr. GoodCat as he so casually puts it is "a world class assassin" but if that were true how would anyone including the detectives in the film had know he was in NYC at the time.
This is the second movie in which Ben Kingsley dies by suffocation with a plastic bag over his head. He commits suicide in the House of Sand and Fog in 2003 using the same method, coincidentally also from the sorrow of losing his son. Although in 'Slevin', it was not suicide, after learning the loss of his own son, he seems to accept his execution as something he deserved for his past crimes.
It is the second movie in which Ben Kingsley is taped to a chair by a character who render him unconscious by hitting him on the skull. The other movie was Death and the Maiden (1994). Both times, it was a revenge for old sins.