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.
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.