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 to filming.
Josh Hartnett (Slevin) lived with writer Jason Smilovic and his girlfriend in New York City 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, Sir 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.
The "Shmoo" that The Boss talks about in the speech that introduces his character are a race of creatures featured in Al Capp's beloved Lil' Abner newspaper comic strip. The Shmoo also gained it's own animated series in the late 1970's with Fred And Barney Meets The Shmoo.
This is the second movie in which Sir Ben Kingsley dies by suffocation with a plastic bag over his head. He commits suicide in House of Sand and Fog (2003) using the same method, coincidentally, also from the sorrow of losing his son. Although in this film, 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 Sir Ben Kingsley is taped to a chair by a character who rendered 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.